A pig-raising neighbor approached us with a story and a theory on phrase origin. His recent attempt at trying to persuade a pig to move against its will had left our friend physically stymied but intellectually challenged. At one point, according to our friend, the pigheaded swine had stood with its forelegs resting against a gate and watched the great wide world beyond. Then, once one of the pig's more powerful hind legs got involved, the pig was off, over the gate and running hog-wild. (来源：英语学习门户 http://www.EnglishCN.com)

So did this little piggie's leg up and over somehow help the phrase a leg up develop the sense "boost?"

Nope. For the origin, we need to think equine, not porcine. Lexicographers believe the figurative leg up comes from the practice of a person creating a human stirrup—cupped hands—into which a would-be horserider places his or her foot for a leg up (and over) a horse. That sense haslegs, as they say, and since the first half of the 19th century, everyone from horseback riders to pig farmers to college professors has been using the phrase to give a leg up to mean "boost" or "give a helping hand to."