a party for a man who is going to get married, to which only his male friends are invited.
On Keith's stag night, his friends left him tied to a lamp-post in Trafalgar Square, wearing only his underpants.
stag night
⇨ stag do; stag night; stagnoun a social event for men only. After STAG PARTYUK, 1965
stag night
A groom's outing with his male friends before his wedding; a bachelor party. Mark knew that his stag night had gotten out of control when the police arrived.