Stands for Outside the Butthole Butt Fuck. Essentially done like a tittyfuck, consists of anal sex with the buttcheeks and no penetration of the butthole
"Dude how'd you like that anal sex?"
"She wouldn't let me penetrate, so I had to settle for fucking her butt cheeks like a titty fuck and cumming on her back"
"Oh you mean an OTBHBF."