Capricious is a formal word that describes someone or something governed or characterized by impulsivity or unpredictability.

// The employees were at the mercy of a capricious manager who always seemed to impose deadlines at the last minute.

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“Tornadoes are among the planet's most fearsome phenomena, with terrifying and capricious wind speeds measured as high as 300 mph. And the U.S. sees more than any other country.” — Doyle Rice, USA Today, 9 May 2024

The adjective capricious and its close relation, the noun caprice (a synonym of whim), both come via French from the Italian word capriccio, which has disputed origins. Capriccio originally referred not to a sudden desire but to a sudden shiver of horror and likely comes from the Italian capo, meaning “head,” and riccio, the word for “hedgehog.” The implication was that someone who shuddered in fear or horror was said to have a “hedgehog head,” meaning that the person’s hair stood on end like the spines of a hedgehog. The link between a whim and a shiver of horror is notably tenuous, though, and a possible link to Italian capra, meaning “goat,” has also been suggested, given the whimsy goats seem to employ in their gamboling. (A full treatment of the disputed etymology can be found here.) Whatever its origins, capriccio came to mean “whim, fancy,” which directly relates to its uses today. Something done in a capricious manner is done on a whim, as in “a capricious decision to join the circus.”