Lewis & Short

1. ăcus, ūs, f. [cf. 2. acer].

  1. I. A needle or pin, as being pointed, both for common use and ornament:“quasarcinatrix veletiam ornatrix utitur,” Paul. ex Fest. p. 9 Müll.
    1. A. Lit.: mirabar vulnus, quod acu punctum videtur, Cic. Mil. 24.
      Hence, acu pingere, to embroider, Verg. A. 9, 582; Ov. M. 6, 23; cf. Plin. 8, 48, § 191; Isid. Orig. 19, 22, 22.
      Esp. a hair-pin: figat acus tortas sustineatque comas, Mart. 14, 24: foramen acūs, the eye of a needle, Vulg. Matt. 19, 24.
      Also, a surgeon’s needle, a probe, Cels. 7, 17.
      Hence,
    2. B. Trop.: acu rem tangere, to touch the thing with a needle; in Engl. phrase, to hit the nail on the head, Plaut. Rud. 5, 2, 19; so, to denote careful and successful effort: si acum quaereres, acum invenisses, id. Men. 2, 1, 13.
  2. II. The tongue of a buckle, Treb. Poll. Claud. 14.
  3. III. I. q. acus, ĕris, Col. 2, 10, 40.
  4. IV. An implement of husbandry, Pall. 1, 43, 2.

2. ăcus, ĕris, n. (also, ūs, f., v. 1. acus, III.) [kindred with acus, ūs, Goth. ahana, old Norse agn, old Germ. Agana], = ἄχυρον, the husk of grain and of pulse; chaff, Cato, R. R. 54, 2; Varr. R. R. 1, 52; 57; 3, 9, 8.

3. ăcus, i, m. [1. acus], a kind of sea-fish with a pointed snout, the hornpike or gar-pike (Gr. βελόνη): acus sive belone unus piscium, etc., Plin. 9, 51, 76, § 166: et satius tenues ducere credis acos, Mart. 10, 37, 6; cf. Plin. 32, 11, 53, § 145, where belonae again occurs. (Some read una for unus in the passage from Plin., and acūs for acos in Mart., as if these forms belonged to 1. acus.)