From Gaelic 'Fearghus' meaning man of vigor or supreme strength, borne by Irish kings.
Fergus is an Irish and Scottish Gaelic name of considerable antiquity, composed of two elements: fear, meaning man, and gus, meaning vigor, strength, or force. Together they yield something like 'man of strength' or 'vigorous man' — an unambiguous declaration of character that suited the warrior culture in which the name arose. It appears in the oldest stratum of Irish mythology: Fergus mac Róich is one of the great figures of the Ulster Cycle, a legendary king of Ulster who surrenders his throne through a ruse involving his foster-son Conchobar mac Nessa and spends the rest of the mythological cycle as an exile of extraordinary dignity and physical might.
He is described as so powerful that it takes seven men to satisfy his hunger, seven women to satisfy his desire, and his sword — the Caladbolg — may be the prototype for Excalibur. Historically, Fergus mac Eirc is credited in Scottish tradition with founding the kingdom of Dál Riata, the Gaelic realm that bridged northeastern Ireland and western Scotland in the fifth and sixth centuries CE, making him a founding figure of Scottish identity and the ancestor of the Scottish royal line. The name thus sits at the intersection of Irish myth and Scottish history, equally at home on both sides of the North Channel.
In contemporary usage, Fergus remains popular in Scotland and Ireland, where it reads as warmly traditional without being archaic. It has a pleasing directness — no ambiguity, no softening diminutive — and the natural nickname Gus gives it surprising versatility. In North America and Australia it is rarer, which gives it the appealing quality of being immediately understood as Celtic without being generic.