GALGACUS, or perhaps rather Calgacus, a Caledonian chief who led the tribes of North Britain against the invading Roman army under Cn. Julius Agricola about A.D. 85 and was defeated at the battle of Mons Graupius (Tac. Agric. 29). The name recurs much later, in Adamnan's Life of Columba, in the name of a wood near Londonderry, Daire-Calgaich or Roboretum Calgachi, "the wood of Calgacus": it may be Celtic and denote "the man with the sword."
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)