acus

• (Classical Latin) IPA^(key): /ˈa.kus/, [ˈäkʊs̠]
• (modern Italianate Ecclesiastical) IPA^(key): /ˈa.kus/, [ˈäːkus]

From Proto-Italic *akus, from Proto-Indo-European *h₂eḱus, from the root *h₂eḱ- (“sharp”). Related to acuō (“sharpen, whet”) and aciēs (“edge”).^[1][2]

acus f (genitive acūs); fourth declension

2. a needle, a pin
3. bodkin
4. Alternative form of acus (“bran”)
   • 4 CE – c. 70 CE, Columella, De re rustica 2.14:

     nam sēmina excussa in āreā jacēbunt, superque eā paulātim eōdem modō reliquī fasciculī excutientur, ac dūrissimae quidem acūs rejectae sēparataeque erunt ā cūdentibus, minūtae vērō, quae dē siliquīs cum fabā resēderint, aliter sēcernentur.

Fourth-declension noun.

From Proto-Italic *akos, from Proto-Indo-European *h₂éḱos (“tip, bristle; ear/blade of grain, awn, chaff”), from the root *h₂eḱ- (“sharp”). Cognates include agna (“ear”) and Proto-Germanic *ahaz (“ear”), Proto-Germanic *aganō, *ahanō (“chaff”) (> English awn), Ancient Greek ἄχυρον (ákhuron) (> Greek άχυρο (áchyro, “hay”)), and Tocharian B āke (“tip, peak, end”).^[1][3]

acus n (genitive aceris); third declension

2. bran, awn, chaff

   Synonym: āplūda

   • 234 BCE – 149 BCE, Cato the Elder, De agri cultura 54.2:

     sī fēnum non erit, frondem īligneam et hederāceam datō. paleās trīticeās et hordeāceās, acus fabāginum, viciam, vel dē lupīnō, item dē cēterīs frūgibus omnia condito.

   • 116 BCE – 27 BCE, Marcus Terentius Varro, Agricultural Topics 1.52:

     īs trītīs oportet ē terrā subjectārī vallīs aut ventilābrīs, cum ventus spīrat lēnis. ita fit ut quod levissimum est in eō atque appellātur acus <ac palea> ēvannātur forās extrā āream ac frūmentum, quod est ponderōsum, pūrum veniat ad corbem.

   • 116 BCE – 27 BCE, Marcus Terentius Varro, Agricultural Topics 1.57:

     parietēs et solum opere tēctōriō marmorātō lōrīcandī; sī minus, ex argillā mixtā acere ē frūmentō et amurcā, quod mūrem et vermem nōn patitur esse et grāna facit solidiōra ac firmiōra.

   • 116 BCE – 27 BCE, Marcus Terentius Varro, Agricultural Topics 3.9.8:

     in cubīlibus, cum parturient, acus substernendum.

Third-declension noun (neuter, imparisyllabic non-i-stem).

• acerātus
• acerōsus

• Ernout, Alfred, Meillet, Antoine (1985) “acus”, in Dictionnaire étymologique de la langue latine: histoire des mots‎^[1] (in French), 4th edition, with additions and corrections of Jacques André, Paris: Klincksieck, published 2001, page 7
• “acus”, in Charlton T. Lewis and Charles Short (1879) A Latin Dictionary, Oxford: Clarendon Press
• “acus”, in Charlton T. Lewis (1891) An Elementary Latin Dictionary, New York: Harper & Brothers
• acus in Charles du Fresne du Cange’s Glossarium Mediæ et Infimæ Latinitatis (augmented edition with additions by D. P. Carpenterius, Adelungius and others, edited by Léopold Favre, 1883–1887)
• acus in Gaffiot, Félix (1934) Dictionnaire illustré latin-français, Hachette.
• Carl Meißner, Henry William Auden (1894) Latin Phrase-Book‎^[2], London: Macmillan and Co.
  • you have hit the nail on the head: rem acu tetigisti
    
• “acus”, in Harry Thurston Peck, editor (1898), Harper's Dictionary of Classical Antiquities, New York: Harper & Brothers
• “acus”, in William Smith et al., editor (1890), A Dictionary of Greek and Roman Antiquities, London: William Wayte. G. E. Marindin