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1.
When a small portion of a group understands an obscure reference that refers to something that the rest of the group is unaware of. This is used expressly when referring to a work situation that other work people do not understand.

Synonym: inside joke
Person #1: Hey, Bruins, agree?

Person #2 (to all): What are you talking about?

Person 3#: Don't worry, it is an in-house joke.
by DDT in BOS June 21, 2011