1
general::
Go unaccompanied by a person of the opposite sex to a social event, as in John decided to go stag to his roommate's party, or Some of the girls are going stag to the dance. Although this term originally applied only to men attending an event without a woman companion, it is now applied to women as well. [c. 1900]
American Heritage Idioms
2
general::
Phrase(s): go stag
to go to an event (which is meant for couples) without a member of the opposite sex. (Originally referred only to males.) • Is Tom going to take you, or are you going stag? • Bob didn’t want to go stag, so he took his sister to the party.
McGrawhill's American Idioms And Phrasal Verbs