acrus

Reassignment of Classical Latin ācer from the third declension to the first/second, facilitated by the fact that the nominative singular ācer can itself be taken as a second declension form, initially yielding a paradigm ācer/ācra/ācrum (compare Classical Latin niger/nigra/nigrum 'black').^[1] The wide distribution of its descendants across the branches of Romance implies that its presence in the oral language considerably predates its appearance in writing.^[1]

ācrus (feminine ācra, neuter ācrum); first/second-declension adjective (Late Latin^[2], proscribed)

1. Alternative form of ācer (“sour, bitter”)
   • c. AD 375, Flavius Sosipater Charisius, Artis grammaticae libri V. 1.117:

     acrum tamen pro acrem Cn. Matius Iliados XV ⟨ut⟩ Vindex a litterae libro I notat

     But 'acrum' for 'acrem' Gnaeus Matius Iliados XV ⟨as⟩ Vindex notes in book 1 of letter A

   • [3rd–4th century, Appendix Probi, line 41:

     acre non acrum

     (The correct form is) acre, not acrum]

First/second-declension adjective.