BCFin' Share definition Flag Stands for butt cheek fucking. Basically means sticking your penis between some chicks butt cheeks and fucking the shit out of her.AKA - titty fucking her back door. That nappy hoe was on her period, so I BCFed her.Hey bob, I'm going BCFin' this weekend. Wanna come?That bitch took a shit when I was BCFin' her. Turned into a fucking chili dog. BCFin' by WikiPete February 14, 2012 Get the BCFin' mug.