According to Tacitus, Calgacus was a chieftain of the Caledonian Confederacy who fought the Roman army of Gnaeus Julius Agricola at the Battle of Mons Graupius in northern Scotland in AD 83 or 84. Some older scholarship has proposed a Brittonic derivation, *calg‑ac‑os, meaning 'possessing a blade' or 'swordsman', but this interpretation is highly speculative and not attested in contemporary sources. Several scholars have connected the name Calgacus with the Gaelic root calg-, meaning "a prickle; the point of a weapon; anything pointed". Related forms recorded in Dwelly's Illustrated Gaelic–English Dictionary include calgach, calg and calgag ("pointer-finger"), all belonging to the same semantic field of pointed or projecting objects. MacBain's Etymological Dictionary of the Gaelic Language also records calg / colg as meaning "awn, bristle; anything pointed; a sword". Whether the word is a name or a title is unknown.