A male who has either suffered a detrimental leg injury on one leg that impedes his mobility either from scar tissue or because he is missing one leg. The probability of there being a one-legged greg are exponentially higher in the modern day society. The female counterpart is known as one-legged Meg.
Greg:"FUCK! My hamstring just locked up, help a brother out."
Jeff: " No problem one-legged Greg, on it!"