Green’s Dictionary of Slang

boon coon n.

[SE boon (companion) + coon n. (5)]

(US black) a close companion.

[US]Hughes & Bontemps Book of Negro Folklore 481: boon-coon : Bosom friend. Stacy’s my boon-coon.
[US]‘Iceberg Slim’ Pimp 75: Your ‘boon coon’ ‘Party’ caught sixty in the county.
[US]E. Torres Q&A 187: That’s cool for you. You his boon-coon.
[US]Eble Sl. and Sociability 81: Rhyming is operative in honky-tonk and boon-coon ‘best friend’.