basement dweller

noun2

1. •
   (informal, offensive) An adult who lives in their parents' house (stereotypically in their basement); often with implications of idleness or social awkwardness.

   • Who Do You Hang With — Basement Dwellers or Penthouse Cheerleaders? It's important to take regular inventory of the people you are surrounding yourself with on a fairly consistent basis. — 2014
2. •
   (informal, offensive) A nerd or geek; a person lacking a social life.