(Noun) The act of digitally penetrating ones partners' anus (and/or rectum) while under the influence of alcohol and/or narcotic(s) under the pretense that it is in-fact the "partners'" vagina yielding a brown ring-like stain encompassing the radius of the offending finger.
"Dude....I totaly "finger banged" Justine last night just before I passed out!!! But when I woke up..... I HAD A FUCKING SMEAGOL!!!!!" GROSSSS!! IT STINKS LIKE YOUR MOM!!!