Daily Chinese Idiom Guessing Game
With justice on one's side, one speaks and acts boldly.
理直理由正确、充分；气壮气势旺盛。理由充分，说话气势就壮。
明·冯梦龙《古今小说》卷三十一便捉我到阎罗殿前，我也理直气壮，不怕甚的。”
He spoke out against the injustice, feeling completely in the right.