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A brainless idiot who messes everything up and acts like a lunatic. You can also call a dog a jack-a-leg if it's doing weird stuff and hates everyone.
My buddy is such a jack-a-leg, he spilled coffee on my boss and then laughed at him.
That dog is a jack-a-leg, it ran into the street and bit the mailman.
My teacher called me a jack-a-leg because I drew a mustache on the principal's face.