a party given a bachelor by his male friends before his marriage, often on the night before the wedding, ranging from a formal gathering to a night of carousing.
3.
a gathering for men only, especially one for the purpose of watching sexually titillating movies or exhibitions.
Origin
1850-55, Americanism
British Dictionary definitions for stag party
stag night
noun
1.
a party for men only, esp one held for a man before he is married Compare hen night, hen party
Idioms and Phrases with stag party
stag party
A social gathering for men exclusively, often involving entertainment considered unsuitable for women. For example, They wanted to give him a stag party before the wedding but John wasn't interested. This idiom uses stag in the sense of “a man unaccompanied by a woman.” [ c. 1850 ]