Bark Up The Wrong Tree

to be mistaken; to have the wrong idea

Examples:

• Bob was barking up the wrong tree when he tried to ask Amy for a date. She only dates rich guys.
• You’ll be barking up the wrong tree if you ask the boss for a raise. This company never gives raises.

Origin: This idiom comes from hunting, and refers to the action of a dog that mistakenly believes it’s prey is in a particular tree. In such a case, the dog is barking up the wrong tree.