The joker in the pack phrase
A person or thing that is different from other things or people and likely to have an unexpected effect on the situation.
He is known as the joker in the pack for quite a long time.
The joker in the pack in this contest will be the weather, but not the participants.
Something that only has one of two different end results: positive or negative
To say something without thinking
Caught by surprise
Used to describe something which is uncertain.
The Joker playing card. (Image Source: Pinterest)
It may derive from a standard deck of playing cards. A joker is an extra card that doesn't belong to any of the four suits (clubs, diamonds, hearts, and spades) and usually has the image of a jester. It's used in poker as a wild card.
Used to allude that the last force, problem or burden which is seemingly minor and small causes a person, system or organisation to collapse or fail
Her husband's violent act last night was the straw that broke the donkey's back and she left him