Lewis & Short

călĭgŭla, ae, f. dim. [caliga],

  1. I. a small military boot; only post-class. ap. Ambros. Ep. 7, 53; Hor. S. 1, 3, 128; Schol. ad Juv. 3, 67.
    Hence,
  2. 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.