The Feldhauptmann (plural: Feldhauptleute) was a historical military appointment, during the time of the Landsknechte or mercenaries in European warfare, who commanded a Fähnlein, a unit of roughly battalion-size. A literal translation is "field captain".
The commander of a regiment, who initially still commanded his own Fähnlein, was designated as the Obristfeldhauptmann ("senior field captain"), the most senior of the Feldhauptleute in the regiment. Orally this was abbreviated to Obrist, the equivalent of "colonel" (likewise the rank of general arose from Generalobrist.)
German dictionary by Jacob and Wilhelm Grimm General