Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
sēmĭ-hōra, ae, f., a half-hour, half an hour, Cic. Rab. Perd. 2, 6; Cels. 3, 21; Petr. 69, 4; Auct. B. Afr. 38.