unknown quantity

Idioms  

1. An unpredictable person or thing, as in We don't know how the new pitcher will do—he's an unknown quantity. This expression comes from algebra, where it signifies an unknown numerical value. Its figurative use dates from the mid-1800s.

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Example Sentences

Mr Barrington said Wolves of Wiltshire have specific licenses that allow them to care for the animals, because they were an "unknown quantity, and they could have potentially required a dangerous animals license".

From BBC • Jan. 11, 2026

Humphries, but the veteran is of late something of an unknown quantity.

From Los Angeles Times • Jun. 20, 2025

Is Levi a professional wildcard—some unknown quantity who requires vetting by the president-elect himself, or someone who demands that his every former employer be contacted by the president-elect himself?

From Slate • Jan. 9, 2025

But when he quit, that number jumped to 9%, reflecting the fact that Harris was an unknown quantity to many Americans.

From Salon • Oct. 29, 2024

Jacqui is an unknown quantity in an unmarked bottle, and we are currently at her mercy.

From "Dry" by Neal Shusterman and Jarrod Shusterman

Related Words

• also-ran
• long shot
• sleeper
• underdog