Originally it came from the joke that went something like this:

Two men were trapped on an island of savages. The chief gives them a choice. You may undergo "snu snu" or you may die. The first man chooses snu snu. A large savage takes him and rapes him violently. Afterwards, the man is free to go. The second man decides he'll take death. The chief smiles and yells out "death by snu snu!"

You:i had some hard snu-snu last night
Friend: oww real thats some dope shit
Noob: im lame i dont know wat ur talkin about

Fred: I want snu snu!
Mike: (punches Fred in the face) you fucking cockloving faggot!