Potatoes from the same furrow
I discovered an interesting Welsh expression today – maen nhw’n datws o’r un rhych (‘they’re potatoes from the same furrow’), which is one equivalent of saying that they are as thick as thieves, i.e. they are close friends. Other Welsh equivalents of this expression include maen nhw’n gryn lawiau (‘they’re pretty (?) hands’); maen nhw’n yng nghegau ei gilydd (‘they’re in mouths together’); and maen nhw’n drwyn wrth drwyn (‘they’re nose to nose’).
In French the equivalent of this phrase is comme larrons en foire (‘like thieves in (a) fair’) – the word larron is a old word for thief – the usual word is voleur.
What about in other languages?