wild card

An unpredictable person or event, as in Don't count on his support-he's a wild card, or A traffic jam? That's a wild card we didn't expect. This expression comes from card games, especially poker, where it refers to a card that can stand for any rank chosen by the player who holds it. The term was adopted in sports for an additional player or team chosen to take part in a contest after the regular places have been taken. It is also used in computer terminology for a symbol that stands for one or more characters in searches for files that share a common specification. Its figurative use dates from the mid-1900s.

You describe someone or something as a wild card when they cause uncertainty because nobody knows how they will behave or what effects they will have. Some Italians think of her as a possible wild card in next year's presidential election. Note: In games such as poker, a wild card is a card that can have any value a player chooses.

An unpredictable person or event. The expression originated in poker and other card games, where a wild card may represent any rank chosen by the player holding it. It first was transferred to sports, for an additional player or team chosen to take part in a competition, and then to computer terminology, for a symbol that stands for one or more characters in searches for files. It has been used figuratively since the mid-1900s. For example, “The flight was canceled? That’s a wild card we didn’t count on,” or “Henry’s always been a wild card—you don’t know which way he’ll vote.”