Merriam-Webster's Word of the Day for June 9, 2026 is:

deleterious • \del-uh-TEER-ee-us\  • adjective

Deleterious is a word used in formal speech and writing to describe something that is damaging or harmful, usually in a subtle or unexpected way.

// Though effective at keeping pests away from plants, the spray is no longer used because of its deleterious effects on the respiratory system.

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Examples:

“Canceling email addresses used by alumni over many years could have deleterious consequences for professional networking, which will become increasingly important as the AI roll-out accelerates and disrupts careers for thousands of college graduates.” — William Golz, NOLA.com (New Orleans, Louisiana), 15 Apr. 2026

Did you know?

When you hold down the delete key on your keyboard or touchscreen, the effect—whoosh!—is instantaneous. Deleterious effects, however, are often not so obvious; deleterious (ultimately from a Greek word meaning “to hurt”) is used to describe things that are harmful in ways that are unexpected, slow-acting, or not readily apparent. Although most often used in formal speech and writing, deleterious is far from rare. It even pops up from time to time in film and television, especially from the mouths of wonky characters, as when Seven of Nine warns the Doctor in an episode of Star Trek Voyager, “The nebula is having a deleterious effect on all the ship’s technology,” or when Higgins exclaims in the original Magnum P.I. series, “It’s shocking what a deleterious effect a regimen of nothing but mushrooms can have on a man.” We’ll take your word for it, Higgins.