I love words, and love finding out how wrong I've been about them (sometimes). For example, I always thought it was "bell weather," and tonight wondered what kind of weather was good for bells.
Alas, it's bellwether, but which is equally interesting. According to Wikipedia, and confirmed in the dictionary,
The term is derived from the Middle English bellewether and refers to the practice of placing a bell around the neck of a castrated ram (a wether) leading its flock of sheep. The movements of the flock could be perceived by hearing the bell before the flock was in sight.
I suppose it could also have been "belle weather," when the lovely ladies lead their men to desired ends.