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

     acre non acrum

     [The correct form is] acre, not acrum

First/second-declension adjective.