"Guybrush Ulysses Threepwood is the main character of the Monkey Island series of computer adventure games by LucasArts. The voice of Guybrush is actor Dominic Armato in the third, fourth and fifth games, as well as the enhanced remakes of The Secret of Monkey Island and Monkey Island 2: LeChuck's Revenge, with the latter scheduled for release in the summer of 2010. Though a mighty pirate by his own account, it is a running joke throughout the games for characters to garble Guybrush Threepwood's unusual name, either deliberately or accidentally, including "Thriftweed", "Peepwood", "Threepwillow", "Gorbush" and "Threekwood". He is also referred to as "Fancy Pants".
The origin of the name "Guybrush" comes in part from Deluxe Paint, the tool used by the artists to create the character sprite. Since the character had no name at this point, the file was simply called 'Guy'. When the file was saved Steve Purcell, artist responsible for the sprite, added 'brush' to the file name, indicating that it was the Deluxe Paint "brush file" for the "Guy" sprite. The file name was then "guybrush.bbm", so the developers eventually just started referring to this unnamed 'Guy' as "Guybrush". Guybrush's surname "Threepwood" was decided upon in a company contest and is derived from P. G. Wodehouse's family of characters including Galahad Threepwood. "Threepwood" is also rumored to have been the name of Dave Grossman's RPG character."
Source: Wikipedia, "Guybrush_Threepwood", available under the CC-BY-SA License.