noun [ sing. ]
1.
a person or thing that is used to hide the real purpose of a particular course of action
2.
a politician who competes against the leader of their party in order to see how much support the leader has; a stronger candidate can then compete against the leader more seriously
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WORD ORIGIN
early 16th cent.: from the former practice of using a horse trained to allow a fowler (hunter of wildfowl) to hide behind it, or under its coverings, until within easy range of prey.