Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
spīnus, i (abl. spinu, Varr. ap. Charis. p. 117 P.), f. [spina], a blackthorn, sloe-tree: Prunus spinosa, Linn.; Verg. G. 4, 145; Pall. Febr. 25, 6; id. Insit. 63.