(essere) come culo e braghe/come culo e camicia
Meanings of "(essere) come culo e ..."
(To be) like an ass and (in) pants/like ass and shirt. Considered a vulgar expression, culo is impolite. The 'like ass and shirt' is often interpreted as 'thick as thieves', or 'hand and glove'. These are things that have a very close relationship with each other, that are always near. The shirttail tucks, goes into the pants too.
Avere una relazione molto stretta, stare sempre insieme