Someone who is being either annoying, inappropriate, or an asshole.
Suck vage, you're being a buttmonster.
by poopcock November 18, 2007
An alternate name for a demon (or "pinky demon") in ID Software's original Doom game series.
Blood and viscera flew everywhere when I attacked the group of butt monsters with my chainsaw.
by Timotheus325 January 21, 2009
An oversized 'butt plug'. See 'Butt Plug'
You, me, the Butt Monster. Saturday night. Be prepared
by MKULTRA July 07, 2013