The involuntary and violent rearward snapping of the head and neck when encountering a horrific smelling egg fart.
Dude 1: (lets huge, stinky fart) Ahhhhh, that felt good. Unfortunately the egg salad sandwich I ate for lunch is coming back to haunt me!
Dude 2: Upon entering same airspace as dude #1, head snapping backward in disgust, starts to cough and gag uncontrollably.
Dude 1: Ohhh, sorry for the fartlash, bro!