The phrase "left hand" refers to one's left arm or hand, and is often used in a figurative sense to mean something that is not reliable, trustworthy, or dependable. This usage comes from an old superstition that the left hand was unlucky because it was believed that demons entered the body through the left foot, so people would use their right hands for important tasks and avoid using their left hands. In modern times, however, this superstition is not widely held, but the phrase "left hand" still carries a connotation of being unreliable or deceitful when used figuratively.