unknown quantity

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.

Example Sentences

The 46-year-old Dutch coach built a fine reputation with successes at Feyenoord, but was an unknown quantity to many Liverpool supporters and untested in the Premier League.

From BBC

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

From Los Angeles Times

Reform are still a relatively unknown quantity in Scotland, and their swift rise to prominence is a factor that could make that election fairly unpredictable.

From BBC

The firm added the crash had caused "multiple explosions" on board and an unknown quantity of jet fuel to be released.

From BBC

In the age of social media and endless football coverage, it is not often a player arrives at a club as a relatively unknown quantity.

From BBC

Related Words

• also-ran 
• long shot 
• sleeper 
• underdog