Green’s Dictionary of Slang

(US) a close friend.

1947		B. Schulberg Harder They Fall (1971) 30: An assistant district attorney [...] was a buddy-buddy of his.
1955		B. Schulberg On the Waterfront (1964) 242: You’re still buddy-buddies with that human meat you think are men.
1956		‘Ed Lacy’ Men from the Boys (1967) 46: Long as I have a buddy-buddy like you, Dewey pal, what have I to worry about?
1958		E. Dundy Dud Avocado (1960) 208: Our old buddy-buddy the Contessa, that perennial glad rag doll.
1965		W. Dick Bunch of Ratbags 52: They all became the best of buddie-buddies.
1976		A. Salkey Come Home, Malcolm Heartland 148: Violence is the buddy-buddy of materialism, they say, in all the good books.