Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.
† trĭgōn, ōnis, m., = τρίγων or τρίγωνον, a kind of ball for playing with, esp. in the baths, Mart. 4, 19, 5; 7, 72, 9; 12, 83, 3: fugio campum lusumque trigonem, a game of ball, Hor. S. 1, 6, 126.