What does "unknown quantity" mean?

A person or thing whose nature or value is a mystery.

Unknown quantity

Common synonyms include: dark horse, designate variable, enigma, frontier, indefinite amount, indefinite quantity.

noun

noun ·Uncommon ·Advanced level

Noun

1
A person or thing whose nature or value is a mystery. idiomatic
"Whatever he had learned of himself was here of no avail. He was an unknown quantity."
2
a variable whose values are solutions of an equation wordnet
3
Used other than figuratively or idiomatically: see unknown, quantity.
"Many of the applications of mathematics require that you identify an unknown quantity, assign a variable to that quantity, and then attempt to express another unknown quantity in terms of that variable."
4
a factor in a given situation whose bearing and importance is not apparent wordnet

"Whatever he had learned of himself was here of no avail. He was an unknown quantity."