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 11, 2010
Get the Underfistingmug.