Following the initial berner (pre boner) and after experiencing climax with a boner, the afterberner is what's left of your penis after sex.
Usually awkward and unwanted the afterberner will leave it's owners with a slightly larger, limp and soft berner.
"Hey Dannytuck that afterberner in man you want your mom to see that?"
"Maybe"
When forcefully and repeatedly thrust your penis into the tailpipe of an ex's car.
Garret: Hey man, did I tell you about that time that my ex cheated on me, so I gave her Honda Civic a Redneck Afterburner?!
Jack: Oh my god, that's hilarious, I did that to my ex after she gave me a windy tuna!