Original: Legit anything that is "real"; for example, a "legit" injury a wrestler might have, a "legit" wrestler has a background in actual fighting, a "legit" event is one that actually took place (outside of kayfabe), a "legit" fight is when two wrestlers actually come to blows.
Simplified: Legit anything that is "real"; for example, a "legit" wrestler has a background in actual fighting, a "legit" event is one that actually took place (outside of kayfabe), a "legit" fight is when two wrestlers actually come to blows.