Having a 'good eye' is connected with generosity and having a 'bad eye' is connected with stinginess.
See this great article on the issue:
I frequently talk about having 'good eye days' when I can see better than usual, and 'bad eye days' when my vision is cloudier and it's much more difficult to see things clearly (after an eye bleed, or new scarring, or a medical procedure has been done).
Guess my eyes are being generous on the good eye days and quite stingy with sight, the little stinkers, on bad eye days. ☺