Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
ructus, ūs, m. [rugo, whence also ructo and erugo], a belching, eructation, rising of the stomach (class. in sing. and plur.): exhalas acidos ex pectore ructus, Lucil. ap. Non. 164, 33: suavis ructus mihist, Plaut. Ps. 5, 2, 9; Cic. Fam. 9, 22, 5; Cels. 4, 5 fin.: foedi pestilentesque ructus, Sen. Ep. 95, 25; Plin. 20, 12, 48, § 122; 20, 17, 66, § 174; Mart. 1, 88, 4 al.