Green’s Dictionary of Slang

(orig. US black) one’s best and most trustworthy friend.

1962		P. Crump Burn, Killer, Burn! 19: You know we’s ace-boon-coons, momma.
1967		P. Thomas Down These Mean Streets (1970) 180: I didn’t want to turn my ace-coon boon against me, but all my life I had wanted to be for real.
1969		B. Beckham My Main Mother 48: You’re my main man, you’re my ace boon coon.
1971		(con. 1930s) D. Wells Night People 71: Say, now you’re my ace boon-koon.
1971		H.E. Roberts Third Ear n.p.: ace (ace boon coon) n. good friend.
1980		E. Folb Runnin’ Down Some Lines 19: Like you got yo’ hope-to-die partner – yo’ ace coon, yo’ main man, yo’ tight-blood and you cain’t git no closer. [Ibid.] 35: One of the closest levels of friendship is reflected in a term like ace and its variants ace coon, ace boon coon, ace coon poon, ace boom boom, a.b.c.
1996		P. Beatty White Boy Shuffle 9: Euripedes and Crispus, his ace boon coon since childhood, sat in a Boston pub.
2007		J.W. Hall Magic City 52: You’re supposed to be my ace boon coon. Have my back.
2013		B. Saranella Unpaid Dues [ebook] You’re my ace boon coon, / my pride and joy, / an ugly motherfucker, / but you'restill my boy.