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]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.