early 14c., "announce, make known in a formal manner" (a sense now obsolete), from Old French denoncier (12c., Modern French dénoncer) and directly from Latin denuntiare "to announce, proclaim; denounce, menace; command, order," from de- "down" + nuntiare "proclaim, announce," from nuntius "messenger" (from PIE root *neu- "to shout").
The negative sense in English developed (probably encouraged by other words in de-) via the meanings "proclaim as cursed, excommunicated, removed from office" (early 14c.); "formally or publicly threaten to do" (1630s); "declare or proclaim to be cursed, wicked, or evil" (1660s). The meaning "make formal or public accusation against, inform against, accuse" (especially in turning on one's co-conspirators) is from late 15c. Related: Denounced; denouncing.