Popularized by the rap group ExMortis, a snackpack is when a woman (usually someone's mother) performs analingus on the rectum of the man while he defecates.
"My ass become a vortex and began to spit faster;
yo mama's face became a national disaster area.
Makin' her face a little scarier,
and my colon drips all night, so you'd better beware"
also
"I ripped off my drawers, and spread my asscrack, and I made that nasty bitch make herself a snackpack"
When a man and a woman are either having sex or just flirting (or anything in between)and the woman assumes a submissive position, most commonly on her knees, then the man jerks-off into the woman's mouth. The woman then swallows his load. Used as a code word in casual conversation and allows for secret requests while in public.
Howard snuck off to give Mary a snackpack at the party.
Blake and Kate laughed when discussing their last snackpack in front of friends.