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.
If someone has a cast iron stomach, they have a strong stomach that can digest unusual food and even bad food without being ill.
Jimmy must have a cast-iron stomach, when he drank milk with lime juice and felt perfectly fine afterward.