To deliver a religious sermon or moral message; to advocate earnestly for a belief or course of action.
From Old French preechier, from Latin praedicare meaning 'to proclaim publicly'. Originally specifically religious, it broadened to any earnest advocacy by the 14th century.
Interestingly, 'preach' and 'predict' share the same Latin root - both involve proclaiming something, whether it's moral truth or future events. The phrase 'preaching to the choir' shows how the word has evolved beyond religious contexts.
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