(slang, vulgar) A jerk; a buffoon; a person who is inept or ill-behaved to the point of being found laughable by others.
2006, Brad Bauer, Hitting in the Clutch, iUniverse (2006), ISBN 9780595395095, page 141:I'm a little loaded, and this assclown behind me has bumped me four times in a row.
2006, Megan McCafferty, Charmed Thirds, Three Rivers Press (2006), ISBN 9781400080434, page 261:"Pretentious, ambitionless assclown," I corrected.
2012, Jef With One F, "5 Horrible Lessons Ender's Game Teaches Kids", Houston Press, 19 June 2012:Card is a raging homophobic assclown who believes that America should rise up violently and overthrow the government for any hint of support of same-sex marriage.