Merriam-Webster's Word of the Day for May 17, 2025 is:

mercurial • \mer-KYUR-ee-ul\  • adjective

Mercurial is usually used to describe someone whose mood changes quickly and unpredictably. It can also describe something that changes frequently, such as weather, or something that is lively and quick, such as someone’s wit.

// The director had some concerns about working with the play’s lead, an actor with a reputation for having a mercurial temperament on set.

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Examples:

“‘The Guiding Light’ was the only radio soap to transition to TV. [Irna] Phillips introduced the ‘cliffhanger’ storytelling device and the mercurial female vixen character who still lives on the small screen today. Think reality TV or a Shonda Rhimes drama.” — Natalie Y. Moore, The Chicago Sun-Times, 3 Apr. 2025

Did you know?

The Roman god Mercury was the messenger and herald of the gods and also the god of merchants and thieves (his counterpart in Greek mythology is Hermes). His swiftness inspired the Romans to give his name to what they correctly assessed as the fastest-moving planet in the solar system. Mercury’s speed also apparently made the name apt for English speakers wishing to describe those whose moods travel quickly between extremes, a meaning mercurial has had since the mid-17th century. The adjective mercurial comes from the Latin mercurialis, meaning “of or relating to Mercury.”