Other forms: legs up
A leg up is an advantage, a head start, or a figurative helping hand. Attending a specialized magnet school might give someone a leg up when applying to certain programs at top-tier universities.
Imagine trying to get on a horse without a stirrup to put your foot in. That would be difficult, so someone might cup their hands together to make a makeshift step. The helper could then push upward, giving you a boost so that you could swing your other leg up and over the horse's back. This physical push to help someone mount a horse is the original meaning of leg up. Now, the term is mostly used figuratively, for anything — mentoring, money, or connections — that helps someone overcome an obstacle or get a head start.