Here's how it evolved:
* Literally: In the past, people would often need help mounting a horse. Someone would stand beside the horse and physically assist the rider by giving them a "leg up," lifting them onto the saddle.
* Figuratively: Over time, the phrase began to be used metaphorically to describe any kind of assistance or advantage that helps someone get ahead or make progress.
So, when we say someone "gets a leg up," we mean they've received something that gives them a head start or an advantage over others.