It is when someone (more than likely a male), takes it in the butt and whoever is on the giving end deposits some semen. the "blow back" occurs when the receipient farts it back out on to the "giver". Hence the term "Blow Back"
I was with my "life partner" going hot and heavy and decided to leave him a gift, he decided to take my "gift" and Blowback on me.
by Arrogant Carroll May 17, 2013