The Rice University Neologisms Database

Etymology : “Stale” as referring to a person is a metaphor for stale food such as bread. Just as eating stale bread is tasteless and boring, a stale person is uninteresting. Used in this case to refer to people who were not actively engaging in a party, and were seen as dull by more lively partygoers. It has a negative connotation, as an uninteresting person is not well liked. This is not a new word, and people, especially older, more conservative people, have been called this as a general description before, but this is the first time I have heard it referring specifically to the context of a party.