You've heard it a million times. Maybe your grandma said it about you and your best friend when you were kids, or you saw it in a tabloid describing two actors who are suddenly inseparable on a press tour. Being as thick as thieves sounds like a compliment. It implies loyalty. It suggests a bond so tight that nothing can wedge a crowbar between the two people involved.
But here is the thing: the phrase wasn't always a "best friends forever" badge of honor.
Honestly, if someone called you this in the 18th century, you might have wanted to check your pockets or look over your shoulder for the constabulary. It didn't just mean "close." It meant "conspiratorial." It was a phrase born out of the gritty, flickering shadows of London’s criminal underworld, where being "thick" wasn't about friendship—it was about survival and silence.
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The Secret Language of Being As Thick As Thieves
Language is weird. It evolves in ways that strip away the original grit. To understand what as thick as thieves actually means, you have to look at the word "thick" itself. Today, we use it to describe a milkshake or maybe someone who isn't the brightest bulb in the box. Back in the 1700s and 1800s, "thick" was synonymous with "intimate" or "crowded together." If you were thick with someone, you were in their inner circle.
Thieves, by necessity, had to be thick.
Imagine the criminal landscape of Regency-era England. You didn't just have pickpockets; you had an entire subculture with its own slang, known as "thieves' cant." This was a private language designed specifically to exclude the "honest" public and the police. If you were a thief, your life literally depended on the person standing next to you keeping their mouth shut. You shared secrets that could get you sent to the colonies or, worse, the gallows.
That is the "thickness" the idiom refers to. It’s a density of shared secrets. It's the kind of closeness that comes from knowing exactly where the bodies are buried—sometimes literally.
The first recorded instances of the phrase started popping up in literature in the early 1800s. Theodore Hook, a man known for his wit and occasional hoaxes, used a variation of it in his 1833 novel The Parson's Daughter. He wasn't talking about two sweet old ladies sharing tea. He was talking about people huddled together, whispering, and clearly up to no good.
Why We Stopped Caring About the "Thief" Part
Why do we use it for bridesmaids and gym buddies now?
Over time, the "criminal" aspect of the idiom softened. It’s a common pattern in English. We take words from the dark side of life and make them cute. Think about how we use the word "wicked" to mean "cool" or "bad" to mean "good." By the time the Victorian era was in full swing, the phrase had started to drift toward general intimacy. It became a way to describe any two people who seemed to have their own private world.
It captures something that "best friends" doesn't.
"Best friends" is public. It’s a status. Being as thick as thieves implies a level of exclusivity that feels almost illicit. It’s the two people at the party who are sitting in the corner, ignoring everyone else, laughing at a joke no one else understands. They aren't just friends; they are a unit. They are a "we" in a world of "me."
Real-World Examples of the Bond
If you look at history or pop culture, you see this dynamic everywhere. It’s not just about liking the same music. It’s about a shared history that creates a barrier against the rest of the world.
- The Wright Brothers: Orville and Wilbur weren't just siblings. They were a closed loop. They argued, sure, but they shared a vision so specific and a language of engineering so dense that even their own family struggled to keep up. They were thick as thieves against the skeptics who thought humans would never fly.
- The Rat Pack: Frank Sinatra, Dean Martin, and Sammy Davis Jr. had a code. They had their own lingo, their own nicknames, and a level of loyalty that meant if you crossed one, you crossed them all. That’s the "thief" energy—the "us versus them" mentality.
- Journalistic Duos: Think Woodward and Bernstein during the Watergate scandal. When you are sitting in a parking garage at 2:00 AM talking to Deep Throat, you aren't just colleagues. You are bound by a secret that could destroy you. That is the peak definition of the phrase.
Is It Ever a Bad Thing?
Kinda. Sometimes.
While we usually use it to describe a "goals" friendship, there is still a lingering shadow to being as thick as thieves. In a workplace, for example, if two managers are described this way, it usually isn't a compliment. It implies favoritism. It suggests that decisions are being made in private, behind closed doors, without the input of the rest of the team.
In psychology, this can border on "enmeshment." This is when the boundaries between two people become so blurred that they lose their individual identities. They start thinking the same, acting the same, and—most importantly—isolating themselves from other healthy relationships.
True "thickness" in a relationship should be a foundation, not a wall.
Common Misconceptions and Similar Phrases
People often mix this up with other idioms, but the nuances matter.
- Birds of a feather: This is about similarity. You both like birdwatching? Great, you're birds of a feather. But that doesn't mean you're as thick as thieves. You don't necessarily have each other's backs in a crisis; you just have the same hobbies.
- Two peas in a pod: Again, this is about being identical. You look alike or act alike. Being thick as thieves is about the bond, not the resemblance. You can be polar opposites and still be thick as thieves because of what you’ve been through together.
- Blood is thicker than water: This is the one everyone quotes wrong. The original (and debated) meaning is "the blood of the covenant is thicker than the water of the womb," which actually means the bonds you choose (like those with your "thieves") are stronger than the ones you're born into. It fits the "thick" theme perfectly.
How to Tell if You and Your Partner-in-Crime Fit the Bill
Are you actually as thick as thieves? Here is a quick litmus test.
It isn't about how long you've known each other. It’s about the "Vault." If you can tell this person something that would ruin your reputation, and you don't lose a second of sleep wondering if they’ll tell someone, you’re there.
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It’s also about the "Silent Conversation." Can you walk into a room, look at your friend, and communicate an entire paragraph of information with a single eyebrow raise? If you have a shared shorthand that makes outsiders feel like they need a translator, you are living the idiom.
The Actionable Insight: Building Deep Bonds
We live in an era of "low-stakes" friendships. We have hundreds of followers and dozens of "mutuals," but very few people we are truly thick with. The irony is that while the phrase comes from the idea of hiding from the law, the feeling of the phrase is what most people are actually looking for in their lives: a sense of unshakeable belonging.
If you want to move a friendship from "casual" to "thick," it requires three things:
- Shared Adversity: You don't have to rob a stagecoach. But you do have to go through something hard together. Whether it’s a grueling project at work or a personal loss, shared struggle is the glue.
- High Trust/Low Judgment: You have to be a safe harbor for their least-polished versions of themselves.
- Vulnerability: You can't be thick with someone if you’re wearing armor. Someone has to lower the bridge first.
The phrase has traveled a long way from the damp alleys of Old London to the captions of Instagram posts. It’s lost its criminal edge, but it has kept its soul. It’s a celebration of the private world two people build together.
To truly honor the origin of being as thick as thieves, focus on the quality of your inner circle rather than the quantity of your social network. Identify the one or two people who hold your "thieves' cant"—the ones who know your shorthand and your secrets—and make a conscious effort to protect that bond. True "thickness" isn't just about showing up for the fun parts of life; it's about the radical loyalty required when you're both "against the world." Use your shared history as a foundation, but ensure your bond doesn't become a cage that prevents you from forming other healthy connections.