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.
Learn the next connected idiom: 理直气壮 (lǐ zhí qì zhuàng) → 壮士断腕 (zhuàng shì duàn wàn)
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