Im a huuuge Monkey Island fan..
Fact(s):
The folks who brought us the Monkey Island games first created a sprite of the character who would be the protagonist. They didn't have a name for our hero yet, so they just saved the sprite as "Guy."
Deluxe Paint, the program they used to create the sprite, appended ".brush" to the end of the file's name, giving us "Guy.brush." Everyone got used to referring to the sprite character as Guybrush, so the name just kind of stuck.
Guybrush got the surname "Threepwood" after the game developers voted on it. Originally, "Threepwood" was the name of one of Dave Grossman's RPG character.