I was reading my Muscle and Fitness today and the back article was called For Whom the Dumbbell Tolls. And this article stated back in the 1500s in Europe the churches had big heavy church bells that were maned by bell ringers. These bell ringers needed to be good at what they id or else they couldv'e been killed so they would practice at home lifting the bell and basically working out with it. You could imagine ringing a bell all through there workout so the cut out the clappers which is the instrument inside that basically makes it ring. So the bells were mute and the word used back then for mute was dumb. Therefore a soundless bell was called a dumbbell.
Just a bit of interesting history for you guys. If I can find the actual article I'll post it up.