Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
acnŭa or acna, ae, f. [ἄκενα or ἄκαινα], a measure or piece of land, 120 feet square, Varr. R. R. 1, 10; Col. 5, 1, 5; cf. Isid. Orig. 15, 15, 5.