Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
† nāpy, yŏs, n., = νᾶπυ (an old form for σίναπι), mustard: sinapi Athenienses napy appellaverunt, Plin. 19, 8, 54, § 171: alterum thlaspi aliqui Persicum napy appellaverunt, id. 27, 13, 113, § 140.