A sexual act. When a man is engaged in sex in the doggy position, he inserts his penis into her (or his) anus and fucks them until he cums. Then withdrawing his penis and wiping it on her (or his) new light colored curtains. It usually ends with the other partner freaking the fuck out.
Dude, the bitch cheated on me, so I pulled a Brown Curtain on her last night.