The Mattis is an act used to seduced a female. The male (usually a bro or tool) begins making out with the female and then proceeds to reach around to her ass and insert a finger into her butthole. This is an attempt to use the Shock and Awe factor for anal stimulation. It is best used in situations when both parties are drunk.
"Dude why'd that girl slap you?"
"I tried to pull a mattis"