Fergus

Means "man of vigour", derived from the Old Irish elements fer "man" and guss "vigour, strength, force". This was the name of several early rulers of Ireland and Dál Riata, as well as many characters from Irish legend. Notably it was borne by the hero Fergus mac Róich, who was tricked into giving up the kingship of Ulster to Conchobar. However, he remained loyal to the new king until Conchobar betrayed Deirdre and Naoise, at which point he defected to Connacht in anger. The name was also borne by an 8th-century saint, a missionary to Scotland.

This is the Old Irish form of the name, as well as the usual Anglicized form of Modern Irish Fearghas or Fearghus.