Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
sēnĭo, ōnis, m. [seni], the number six, a sice upon dice: talis jactatis, ut quisque canem aut senionem miserat, August. ap. Suet. Aug. 71; Pers. 3, 48; Mart. 13, 1, 6.