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 Danny tuck that afterberner in man you want your mom to see that?"
"Maybe"
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!