The untimely incident when a male is having unprotected sex with his partner through the precut hole in his boxers, and at the point of ejaculation pulls out not only of his partner but back into his boxers thus, ejaculating into his underwear. The soiling of one's boxers from the inside with seminal fluid is the Rusty Flapjack.
“So, I was fucking this bitch hard from behind in my Ferrari, right? I'm saying balls deep, you know? and right before I busted my nut, I thought about the leather and pulled a rusty flapjack. Fuck! I shoulda nutted on that tight, young ass!”