Stereotypical home for the geek, nerd, fatbeard, loser, etc. The term implies that the individual still lives with one or both of his parents despite being a grown man. Reasons for still living at home may include a lack of drive and ambition, being unable to afford a place of his own due to spending all of his money on his hobbies (i.e. Star Trek figurines, comic books, and online role-playing games), or just plain being a mama's boy.
"Jeff is such a loser. He really needs to move out of his mom's basement and work on getting laid rather than spending every free minute playing World of Warcraft."