fitb

"Finger in the butt." Generally used as a positive remark OR negative remark depending on how you view the act itself.
The act of slipping your finger in a girl's anus while having sex doggy-style. It can also be done as an act on its own without the intercourse part.

Example 1
Dude 1: "Bro, check out that hot babe!"
Dude 2: "Oh yeah, FITB!"

Example 2
Dude 1: "Bro, this sandwich is awesome!"
Dude 2: "Yeah bro, FITB!"

Example 3
Dude 1: "Bro, I just got kicked out of college"
Dude 2: "Shit bro, FITB!"