unknown quantity

Idioms & Phrases

unknown quantity

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

The American Heritage® Dictionary of Idioms by Christine Ammer.
Copyright © 1997. Published by Houghton Mifflin.
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WordNet
unknown quantity

noun
1. a factor in a given situation whose bearing and importance is not apparent; "I don't know what the new man will do; he's still an unknown quantity" 
2. a variable whose values are solutions of an equation [syn: unknown
WordNet® 3.0, © 2006 by Princeton University.
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