What does the proverb mean - It's an ill wind that blows no good?

The complete expression is "blows no one good." It means there is very rarely an event no matter how bad that doesn't benefit somebody. Even in a disaster like an earthquake builders for instance, will benefit in all the reconstruction work. So it is very bad thing to happen that somebody does not benefit (an ill wind).

Centuries ago naval battles involving great warships under sail were common. During a battle these massive sailing ships would maneuver for best position. It could be a slow process and wind direction not only had to be determined but if winds were light, wind direction had to be predicted well in advance. If the wind shifted unpredictably it would often be at the expense of some warring ships but to the benefit of others. Thus the proverb.