From Longman Dictionary of Contemporary Englishbosom friend/buddy/palbosom friend/buddy/palliteraryFRIEND a very close friend → bosom
Examples from the Corpus
bosom friend/buddy/pal• It is now that man contemplates, for it is now that the sea is a bosom friend.• It is an open secret that he and Reg Pybus are bosom pals.• Queequeg says they are now married-meaning they are bosom friends who would defend each other to the death.• There was less arguing after that, but the two actors never became bosom buddies and never worked together again.• He was still my friend, my bosom friend.• The first was affection of the kind that binds families together, or bosom pals.• Benny and Garry were bosom pals once again. 