Lewis & Short

1. obsĭdĭum, ĭi, n. [obsideo], a siege, investment, blockade (mostly ante-class. and post-Aug. for obsidio; not in Cic. or Cæs.; but cf. obsidio).

  1. I. Lit.: obsidium dictum ab obsidendo, quominus hostis egredi posset inde, Varr. L. L. 5, § 90 Müll.: obsidium, tam quam praesidium, subsidium, recte dicitur, Paul. ex Fest. p. 193 Müll.: saevo obsidio premere aliquem, Enn. ap. Non. 216, 29 (Ann. v. 28 Vahl.): obsidium facere Ilio, Plaut. Bacch. 4, 9, 24: obsidio circumdare, Tac. A. 13, 41: obsidium urgere, id. H. 4, 28; Flor. 4, 4, 4; Gell. 15, 31, 1; Amm. 20, 7, 3: ad liberandum Mogontiaci obsidium, Tac. H. 4, 37.
  2. II. Trop.
    1. A. A waylaying, an ambush: obsidia hominum aut insidiosorum animalium, Col. 8, 2, 7.
    2. B. Attention, foresight: curatoris, Col. 9, 9, 1; cf. obsidio.
    3. C. Danger: tuo tergo obsidium adesse, Plaut. Mil. 2, 2, 64.