Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
Ātrŏpŏs, i, f., = Ἄτροπος (not to be turned), one of the three Parcœ, Mart. 10, 44, 6; Stat. S. 4, 8, 18; id. Th. 3, 68.