sticking an object (penis, vibrator, etc.) in one's ass. Some guys love it cause it's tight, or maybe cause the girls vagina is too loose. Some girls love it for reasons unknown to me, but most likely because it feels good.
Contrary to popular belief, it is NOT mostly done to women. Everybody does it. (Even some straight guys like things in their ass. Males do have a g-spot...and it's up there.)
Me: Man, that red-headed bitch Natalie had the loosest pussy EVER! So I had anal sex with her...unfortunately, since her best friend kept fucking her in the ass with a massive vibrator, her asshole was loose too!
Friend: Man, that sucks...
Me: Yeah. I'm glad I wore a condom. I found out she slept with lots of guys, including our old friend Roman. It turned out she got some kinda STD, Herpes or something.
Friend: Wow...well, that's what the bitch got for sleeping with her boyfriend's best friend, and then lying to everyone about it.