Refers to the standard porn-movie sequence of switching between anal and vaginal penetration, and finally ending with a blowjob and/or facial cumshot.
However, the acronym is best used on a webforum as a secret was of claiming a girl looks dirty (like a porn star)
The amount of "AM"'s in the acronym can vary - more meaning a dirtier looking girl!
Forum Guy#1: AMAMAMAMATM
Forum Girl: WTF??
Gorum Guy#2: LOL!