What is "calgacus"

thumb|upright=1.2|19th century print depicting Calgacus delivering his speech to the Caledonians. According to Tacitus, Calgacus (sometimes Calgacos or Galgacus) 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. His name can be as interpreted as Celtic *calg-ac-os, "possessing a blade", and is seemingly related to the Gaelic "calgach". Whether the word is a name or a given title is unknown.