A general name for a guy who makes disgusting jokes about butts, assholes, shit, etc. as well as sexual acts relating to butts and or shit. Marcbutt also knows the name of obscure sexual acts. Can be used as a term of endearment if you enjoy Marcbutt's jokes and knowledge.
A chick gave me a stingbug last night when we were fucking and it reminded me of Marcbutt.