(noun) In a convertible it's the (somewhat surprising) way the air blows from the back of the car toward the front. Many convertible drivers wear hats to reduce the effects of blow back. Fancy convertible makes such as Mercedes, BMW and Porsche often mount a screen behind the car's front seats to reduce blow back to those passengers.
Red thought he was all cool in his convertible until the blow back pushed his cap over his eyes and he ran down that mailbox.
When you fart into a cushioned chair or seat, get up for something and then sit down again, only to have your weight cause the cushion to blow the offensive odor back into your face.