1. The act of assisting someone, giving someone a boost. For example, Studying with Jane, who knows French history well, will give you a leg up for the final exam. This usage alludes to helping a person get on a horse by getting a foot in the stirrup. [First half of 1800s] 2. A position of advantage, as in Because of the advertising campaign, we had a leg up on the competition.
American Heritage Idioms