Something outside of the normal rules, category, etc; an unpredictable thing, event, etc: Being from Princeton wasn't like being from Jersey, it was a wild card(1920s+ Card games)
A team picked for a playoff by some more or less arbitrary method, not having won its championship during the season: We can always hope the Lions will be the wild card(1950s+ Sports)
[fr poker and other games, where in some cases one or more wild cards, having any value the player desires, may be designated]
The Dictionary of American Slang, Fourth Edition by Barbara Ann Kipfer, PhD. and Robert L. Chapman, Ph.D. Copyright (C) 2007 by HarperCollins Publishers. Cite This Source
Idioms and Phrases with wild cards
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.