copains comme cochons

copains comme cochons is masculine or feminine depending on context in French.

thick as thieves

Literally, “friends like pigs”. The expression cochon 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 soçon (“friend”) (or a variation such as the 19th century Parisian chochon) or cosson (“merchant”) might have been altered into a more common word when they fell out of use.