wild card

wild-card的词性：
n.（名词）

1. Games A card assigned specific values that could vary during a game and acquire any value assigned by its holder.
【游戏】 百搭牌，变牌：有特定数值的牌，在游戏中牌值可变化并具有持牌者所指定的任何牌值

2. Slang An unpredictable or unforeseeable factor:
【俚语】 未知的因素，无法预言的因素：

A surprise witness proved to be the wild card at the trial.
审判中有意外的证人也未可知