Where does “hag do” come from?

hag do (English) comes from English hen do, from English hen, from Middle English hen, from Old English henn, from Proto-West Germanic *hannju, from Proto-Germanic hanjō, from Proto-Indo-European keh₂n- — to sing.

Ancestry of “hag do”, step by step

hag do traces back through two separate lines of ancestry.

via English hen do

StepLanguageWordMeaning
1Englishhen doA hen party
2EnglishhenA female chicken (Gallus gallus), especially a sexually mature one kept for her eggs
3Middle Englishhenhen, chicken; female bird
4Old Englishhennhen
5Proto-West Germanic*hannjuhen
6Proto-Germanichanjōhen
7Proto-Indo-Europeankeh₂n-to sing

via English stag do

StepLanguageWordMeaning
1Englishstag doA stag party
2Englishstag partyA bachelor party; a party held for a man who is...
Every word from Proto-Indo-European keh₂n-Every word from Proto-Germanic hanjōEvery word from Proto-West Germanic *hannju