The word "gilla" has its origins in Old Norse, which was spoken by the Norsemen and Vikings in the Scandinavia region during the Viking Age (late 8th to late 11th centuries). In Old Norse, the word was "gildi" or "gildr" and it meant "a servant" or "a companion".
Over time, the word "gildi" or "gildr" made its way into the Old Irish language, where it transformed into "gilla" with the same meaning of "a servant" or "a young male attendant". Old Irish was spoken in Ireland and Scotland between the 6th and 10th centuries.
From Irish, the word "gilla" entered the Middle English language, eventually evolving into "gillie" or "gilly".