Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
imprō̆prĭus (inpr-), a, um, adj. [2. in-proprius], not befitting a thing, unsuitable, improper (post-Aug.): nec improprium nec inusitatum nomen, Quint. 8, 4, 16; cf. cognomen, Plin. 37, 8, 37, § 116: verba, Quint. 8, 3, 57: tropus maxime improprius, id. 8, 6, 37.
Neutr. as subst.: impropri-um, i, something improper, impropriety, faultiness: ei (proprietati) contrarium est vitium; id apud nos improprium, ἄκυρον apud Graecos vocatur, quale est: Tantum sperare dolorem, Quint. 8, 2, 3: sciam us, nihil ornatum esse, quod sit improprium, id. 8, 3, 15; 1, 5, 46.
In plur., Quint. 1, 8, 13; 2, 5, 10; 10, 3, 20; 12, 10, 42.
Adv.: im-propriē, improperly, Plin. 8, 10, 10, § 29; Gell. 6, 6, 2; 17, 1, 1.