Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
impostūra (inp-), ae, f. [impono, II. B. 3.], deceit, imposture (late Lat.), Dig. 47, 20, 3; Treb. Gall. 12; Coll. Mos. et Rom. Leg. 2, 1.