Origin of fenian
- From alteration (influenced by féne body of freemen under early Irish law) of Irish Gaelic fianna bands of young warriors from Old Irish fíanna pl. of fían
From American Heritage Dictionary of the English Language, 5th Edition
- Blend of Old Irish feinne or fianna, plural of fiann (“soldier”), the name of a legendary band of Irish warriors, and Fene or Féni, legendary settlers of Ireland. First attested from 1816.