Douglas Harper's Etymology Dictionary
Wiktionary
leg up
n. 1 (context literally English) the act of assisting another's progress over a wall or other obstacle by forming a step for one of their feet with one's hands 2 (context figuratively English) A boost, a sudden improvement; an advantage 3 (context figuratively English) assistance, help given to get started or overcome a problem or obstacle
Usage examples of "leg up".
Instead, he made a hole through my ankle, pushed a stainless steel bar through it, attached it to a stainless steel horseshoe, and hung my leg up in the air over my hospital bed.
I folded my empty trouser leg up over the stump of my leg and tied it there with my tie.