What it means
Two people are thick as thieves when they’re inseparable, finishing each other’s sentences and always taking each other’s side. It’s usually affectionate, like proper best mates, but it can also carry a sly vibe that they’re whispering, gossiping, or cooking up a bit of mischief together. You’ll hear it about couples, siblings, or any unstoppable double act.
Usage examples
"They’re thick as thieves, those two: nipped out for a pint and came back an hour later with crisps and a mystery traffic cone."
"Those two have been thick as thieves since primary school, you never see one without the other."
"The new starter and the manager are suddenly thick as thieves, which has the rest of us wondering."
"Mia and Shan are thick as thieves lately, always disappearing for ten minutes and coming back giggling."
"You put those two on shift together and they go thick as thieves, sharing snacks and chatting bare nonsense all night."
Where it comes from
The phrase is old and well established in English, showing up by the early 1800s and likely a bit earlier in speech. Here, thick means closely bound or packed together. Thieves were imagined as secretive partners who stuck tight and trusted each other, so the line came to mean two people who are extremely close.
