Caligula, An Emperor called also Caius, who succeeded Tiberius, and was sonne of the noble Germanicus, and hadde the name Caligula, ginen to him by the men of warre which were with his father, because he being a child in the campe, did weare little greaues, or harnesse bootes, called Caligæ. For his life looke Suetonius & Sabellicus.
Lewis and Short: Latin dictionary
călĭgŭla, ae, f.dim. [caliga], a small military boot; only post-class. ap. Ambros. Ep. 7, 53; Hor. S. 1, 3, 128; Schol. ad Juv. 3, 67.—Hence, II. As nom. propr.: Călĭgŭ-la, ae, m., a cognomen of the successor of Tiberius, since from his earliest youth he was engaged in military service; cf. Suet. Calig. 9; Tac. A. 1, 41; 1, 69; Aur. Vict. Caes. 3.