Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
ostrĕātus, a, um, adj. [id.; qs. covered with oyster-shells; hence, transf.], rough, scabby: quasi ostreatum tergum ulceribus (i. e. verberibus), Plaut. Poen. 1, 2, 186.