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1. A deranged relative of a celebrity kept under locks by said celebrity's family and agents. See Solange Knowles for an example.

2. A dude or a chick you are hooking up with on the side, but you might have a main bitch you are trying to keep or else you are looking for some other ass because that dude/chick just ain't cutting it. Thus, you make them your basement baby, hidden from view but still an easy, accessible lay.
Joe won't kiss me in public, I think he's trying to make me his basement baby.

Yo, Beth is hot and all but her personality sucks ass, probably gonna make her a basement baby.
by Edith Wharton March 28, 2011
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A person that spends all his time in his mom's basement playing computer games, must not have a job and be aged over 40!
My Brother is a basement baby. Jake coined the term in adventure time, so its true.
by Bishnuu October 19, 2015
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