Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
tergīnum, i, n. [id.], a hide, raw-hide, as a scourge, Lucil. ap. Non. 414, 9: vostrum durius tergum erit quam terginum hot meum’st, Plaut. Ps. 1, 2, 19.