The ultimate put down. Someone who has all the redeeming features of a rhinos rear end. Derived from a combination of the phrases arse (meaning backside) and clart (meaning a person).
Bill: "I can't make it to the pub tonight"
Karl: "Oh, you arseclart!"
"Oh no, that arseclart just threw up in a taxi! What a mess!"
Famous arseclarts include George W Bush, David Cameron and Dido.