A combination of Norwegian, Welsh and French origin. Closely related to the name of Dragoo, which is a surname for Jernigan, refers to soldiers. The last name took on usually by French soldiers, that happened to be mounted, but not calvary.
Earliest record found in Dooms Day Book of William the Conquerer, as Jernigen, then as Jerniham,then as Jernigan. Refered to a Duke that had aquired lordship by a string of successful military campaigns.