Lewis & Short

Parsing inflected forms may not always work as expected. If the following does not give the correct word, try Latin Words or Perseus.

ăgōn, ōnis, m., = ἀγών, gen. ῶνος a contest or combat in the public games: gymnicus, Plin. Ep. 4, 22: non esse restituendum Viennensibus agona, id. ib. 4, 22 fin.: musicus, Suet. Ner. 22; so id. ib. 23.
Hence, prov.: nunc demum agon est = νῦν γάρ ἐστιν ἀγών, now we must act, now is the time for action, Suet. Ner. 45.