1 definition by The fun guy.
The act of lifting ones rigt leg, during anal activity, and proceeds to continously pump thier arm under the genetalia into the vagina or anus of thier partner.
Guy 1: dude how was last night with brenda?
Guy 2: great! i was underfisting her all night, she loved it!!!
Guy 1: Sweet!!
Guy 2: great! i was underfisting her all night, she loved it!!!
Guy 1: Sweet!!
by The fun guy. July 10, 2010