idiom ~ break a leg

i have found different interpretations for the idiom " break a leg ".
e.g.
form yahoo dictationary : 詛咒某人倒霉, 希望某人交厄運
from electronic dictationary : 祝演出順利
actually, i would like to ask what the real explanation for the idiom is.
Does it mean GOOD LUCK??

thanks for your help~