Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
3. sponsus, ūs, m. [spondeo], an engagement, betrothal; bail, suretyship (rare but class.): quod sponsu erat alligatus, Varr. L. L. 6, § 71 sq. Müll.: de sponsu si quid perspexeris, Cic. Att. 12, 19, 2: agere cum aliquo ex sponsu, Varr. 1. 1.: sponsu locare, Phaedr. 1, 16, 1; Sulp. ap. Gell. 4, 4, 2: lex Furia de sponsu adversus eum, qui, etc., Gai. Inst. 4, 22.