Lewis & Short

bulla, ae, f. [root vhal-; Gr. φαλ-; cf. φαλλός, φύλλον], any object swelling up, and thus becoming round; hence,

  1. I. A waterbubble, bubble: ut pluvio perlucida caelo Surgere bulla solet, Ov. M. 10, 734: crassior, Mart. 8, 33, 18; Plin. 31, 2, 8, § 12; App. M. 4, p. 145, 7.
    Hence,
    1. B. Trop., a bubble, trifle; vanity: si est homo bulla, eo magis senex, Varr. R. R. 1, 1, 1; Petr. 42, 4.
  2. II. Any thing rounded by art.
    1. A. A boss, knob (upon a door, etc.): jussine in splendorem dari bullas has foribus nostris? Plaut. As. 2, 4, 20: bullas aureas ex valvis, auferre, Cic. Verr. 2, 4, 56, § 124 (by such door-studs fortunate or unfortunate days were designated, Petr. 30, 4).
    2. B. A stud in a girdle: notis fulserunt cingula bullis Pallantis pueri, Verg. A. 12, 942; 9, 359; Aus. Cup. Cruc. 49; Prud. Psych. 476.
    3. C. The head of a pin in the water-clock, Vitr. 9, 6, 9 sq.
  3. III. Esp., the bulla, a kind of amulet worn upon the neck (mostly of gold), orig. an ornament of the Roman triumphers, in imitation of the Tuscan kings and Lucumones (Plut. Romul. 25; Fest. s. v. sardi, p. 252), but in the more brilliant era of the Romans worn by noble youths, Cic. Verr. 2, 1, 58, § 152 (cf. Ascon. in h. l., acc. to whom bullae of leather were hung upon the necks of the children of freedmen); it was laid aside when they arrived at maturity, and consecrated to the Lares, Pers. 5, 30; cf.: Lares bullati, Petr. 60, 8; acc. to Plin. 33, 1, 4, § 10, first hung by Tarquinius Priscus upon the neck of his son; cf. also Macr. S. 1, 6, 9 sqq.; Plaut. Rud. 4, 4, 127; Liv. 26, 36, 5; Prop. 4 (5), 1, 131; Suet. Caes. 84; Flor. 2, 6, 24.
    From the Etruscan custom, called Etruscum aurum, Juv. 5, 163.
    Hence the phrase bullā dignus for childish: senior bullā dignissime, Juv. 13, 33.
    It was also hung upon the forehead of favorite animals, Ov. M. 10, 114.