Green’s Dictionary of Slang

(US black) a close companion.

1958		Hughes & Bontemps Book of Negro Folklore 481: boon-coon : Bosom friend. Stacy’s my boon-coon.
1969		‘Iceberg Slim’ Pimp 75: Your ‘boon coon’ ‘Party’ caught sixty in the county.
1977		E. Torres Q&A 187: That’s cool for you. You his boon-coon.
1996		Eble Sl. and Sociability 81: Rhyming is operative in honky-tonk and boon-coon ‘best friend’.