Acronym stands for: "And then my asshole fell out". This is a modified version of the phrase "And then I found 20 bucks". ATMAFO is used to revive any waning story. This is derived from an actual condition where a weightlifting man's anus fell out of his body during a weightlifting competition (yes, google it). The word acronym version is reserved for family friendly social interactions. This is still funny. The long version is much more effective at making the most out of a boring story in front of the appropriate audience.
So yeah I just called a cab and went home. Maybe I will just have the doc look at it in a few days. So yeah... that's pretty much it... oh, And Then My Asshole Fell Out! True story. Note: Substitute ATMAFO once the meaning is well established.