Lewis & Short

campĭ-doctor (better as two words), ōris, m. [campus], one who drills or exercises soldiers in the Campus Martius, a drill-master, Veg. Mil. 3, 6; 3, 8; 1, 13; Lampr. Alex. Sev. 53; Amm. 15, 3, 10; Inscr. Orell. 1790; 3495.