Green’s Dictionary of Slang

(US) a very close friend.

1947		C. Willingham End as a Man (1963) 157: That ought to get the credit away from all the other gangs, the Bunghole Buddies included. [Ibid.] 75: Last year we didn’t do a damn thing. Then the Bunghole Buddies come along with that stuffed bobcat.
1949		J.H. Burns Lucifer with a Book 121: I’ve been noticing you’re bunghole buddies with the one boog in this dump.
2002		The Hangout ‘Revenge’ 🌐 With some women, I could actually hit bottom when I shoved it all the way in. But not with my little bunghole buddy, my own personal scumbag, the boss’s precious little son!!!