I could whip you with one arm tied behind my back

I could whip you with one arm tied behind my back is a phrase commonly used to express how easy it would be to do something; so easy in fact, they can do it with just one hand. By saying they can whip you with one hand tied behind their back, this person means whipping or beating you would be so simple a task they dont need both hands to do it and can have just one working hand.