1
general::
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.
American Heritage Idioms