Prior to its usage as a term for a pre wedding party “hen party” was used in the United States as a general term for an all female gathering usually held at a hostesses residence. In 1897 it was noted that “hen party” was a “time honored idea that tea and chitchats, gossip smart hats, constitute the necessary adjuncts to these particular gatherings”. In 1940 it was described as hosting a Christmas time “hen” party for cabinet wives and “ladies of the press”
Although the practice of giving a party to honour the bride-to-be goes back for centuries, in its modern form, the bachelorette party may have begun during the sexual revolution of the 1960s. It was uncommon until at least the mid-1980s, and the first book on planning bachelorette parties wasn’t published until 1998. Its cultural significance is largely tied to concepts of gender equality.
Initially, parties in honour of the bride-to-be that were labeled as bachelorette party often involved displays of sexual freedom, such as trading intimate secrets, getting drunk, and enjoying male strippers. Parties that honored the bride-to-be without these elements avoided that label. Now the term is used for a wide variety of parties.
Cited from Wikipedia.com