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1.
1 (Noun): A brief, session of sexual intercourse, where both parties climax within 5 to 15 minutes. The purpose of a fuck-snack is to relieve sexual tension to a managable level, and hold the participants over until such time that the parties can engage in proper coitus.

2 (Noun): A short slam fuck.
After sexting all morning, the sexual tension between Sandy and Tim was palpable; therefore, they had no choice but to share a fuck-snack, in the copy room, over their lunch break.
by The Real G Man February 22, 2013
 
2.
(adj) not even worth using the adjective "fucking" to describe the noun, less than fucking worth it.
Look at all those fucksnack assdaggars.
by Fucknugget Joe July 11, 2003