The thing that happens when you speak well before thinking. Or anyone that thinks the UW Badgers mascot is a racoon or a squirrel. Also any person having a crave for watermelon as the majority of their brain.
Person 1:Did you hear what she just said?
Person 2:Yeah what an A-Tom!!
Similar to regular ass to mouth ATM, although this would be normal under orgy situations, ATOM or A.T.O.M. refers to fucking the first girl in the ass then hooking up with a different girl a short time after and receiving a blowjob first.