A condom...something you use to cover your bone. Patented by famous hip-hop artist Devin the Dude in his song "Show 'em," which is about love. It's a play on words and explains clearly what it's used for. Like catching your semen when you "done cum," a Southern expression.
Somewhere in Houston:
Devin's homeboy: We 'bout to lay into deez ho's.
Devin: Got tha duncoms?
Devin's homeboy: Fa sho...gotsta keep ya shit clean.
Devin: Straight...word, <etc...and other rap lingo>
Somewhere in Alabama:
Girl: Oh, oh...Why'd you stop?
Dude: I done cum baby...now I wanna watch me some football.
Girl: Roll tide!