A degrading term generally used at the expense of a friend or foe. By representing the "heesham" you are embodying the lowest of the low. In essence, calling someone "heesham" is like saying: "yo, you stink. you're the lamest of the lame - and you stink."
Brian: Hey man, I can't come out tonight because I've got to program with my n3rdz until the break of dawn.
Zack: Yo, you da heesham.