Wag means to move something back and forth or side to side, usually quickly and repeatedly. When a dog wags its tail, the tail swings rapidly from one side to the other, showing excitement or happiness. You might wag your finger at someone to scold them playfully, moving it back and forth in the air.

The movement is typically short, quick, and energetic rather than slow and sweeping. A cat's tail might wag when the cat is annoyed or hunting, while a dog's wagging tail usually signals joy (though not always). People say “the tail wags the dog” when something small or less important controls something bigger, like when a minor detail derails an entire project.

As a noun, a wag can also mean someone who makes jokes and witty comments, especially clever or mischievous ones. If your friend is always cracking jokes in class, a teacher might call them “quite the wag,” mixing amusement with mild exasperation. This usage is less common but still appears in books and conversation.