(Will appear in the June 2009 issue of Community News)
Something that indicates a trend or leads the way. For example, in an election, people pay attention to certain states that they feel indicate how the country as a whole will go. Those states are bellwethers for the election. You'll sometimes see this word misspelled bellweather. Perhaps some people think that it has to do with a bell that warns of bad weather. Nope. In old English usage, a castrated ram was called a wether. English farmers would put a bell around the neck of a wether and let it lead a flock of sheep. That way, a farmer could find his sheep by following the sound of the bell on the neck of the bellwether. I suppose no farmer in his right mind would try to put a bell around the neck of an uncastrated ram.
I'm collecting all of these at: http://www.dvorkin.com/davidsdefs.html