Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
† ŏpisthŏtŏnĭcus, a, um, adj., = ὀπισθοτονικός, laboring under opisthotony (postAug.): sanat opisthotonicos, Plin. 20, 18, 75, § 197.