A weinerbutt nelson is a person that creeps in your room for short periods of time and is not officially part of your crew yet. Weinerbutt nelson is also when you get fucked in mario kart by stray greens, your own reds, and other miscellaneous bull shit.
Weinerbutt nelson can be used as a noun, verb, adjective, adverb. Can also be abbreviated weinerbutt.
Quit weinerbutting around.
I just weinerbutted all over your face.
Andy Collins is a huge weinerbutt nelson.
That stray green just weinerbutted the shit out of my bitch ass.