copains comme cochons

Etymology
lit. 'friends like pigs'. The expression in this expression is possibly not linked to the actual animal, as it is the basis of many negative expressions in French, and not normally considered a social animal. It has been proposed that older words like (or a variation such as the 19th century Parisian ) or  might have been altered into a more common word when they fell out of use.

Adjective

 * 1)  thick as thieves

Usage notes

 * Also used in the singular, though less commonly: see.