(as) thick as thieves British simile
Be very close friends who can share each other‘s secrets
Ben and Mike have known each other for 20 years and that is the reason why they always look thick as thieves.
The children have been as thick as thieves for weeks, which is very different from the first days of school.
Sophie and I used to be thick as thieves. I don’t understand why she stabbed me in the back.
The idiom first appeared at the end of the 18th century. The word thick used in the idiom (as) thick as thieves means intimate, familiar, closely allied, sharing confidences. The word thieves conjures up the connotation of two people who conspire together in secret, two people whose association is close and excludes the general population. By 1833 the phrase as thick as thieves was referred to as a proverb, intimating that it was well known in spoken English.
To make love or have sex with someone
They just jumped each other’s bones after one date.