"Walt screamed in horror and ecstacy as his anus erupted in orgasm. Without warning, the nose cone-shaped dildo exploded into the air and struck Juanita directly in her left eye, horribly blinding it forever. It was another tragic case of blowback."
When you have to hold in a fart due to being in a public or enclosed space (airplane, subway, car) and the fart rumbles back up into your plumbing. Over the course of several hours, a serious queue of un-expelled farts will build up, prompting an extended cropdusting at the earliest convenience.
Joey: Dude, that 8hour flight from Frankfurt was murder. I musta had about 40 blowbacks.
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.
For example, you go out and eat mexican food chasing it down with the few beers. You wake up the next morning and go to bathroom. The gassy build blows crap on the back part of the toilet bowl called BlowBack.