Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
† acra, ōrum, n., also ae, f., = ἄκρα, a promontory or headland, App. de Mundo prooem.: Acra Iapygia, a promontory in Magna Graecia, Plin. 3, 11, 16, § 100.