dumbbell
The explanation given at the Take Our Word For It website, here (scroll down to the second entry), is partially corroborated in a print source called I've Got Goose Pimples: Our Great Expressions and How They Came to Be by Marvin Vanoni:
In the Middle Ages, bell ringing was considered an art. Bell ringers practiced for many hours and their noise often irritated the public.
Some unknown craftsman solved the problem by inventing a rope mechanism for beginning bell ringers. They went through all the same motions but pulled "dumb bells" -- different size weights -- rather than real bells. This silent invention gace the apprentice ringer plenty of exercise. So when the early health fads started up, people called their exercise weights dumbbells.
p. 80
The corroboration for Take Our Word's claim that "dumbbell" had come to mean "stupid" before the 1920s is here, in this history of the comic strip "Joe Palooka", whose creator originally named it "Joe Dumbelletski". The origin of the American slang word "dumb", for "stupid", apparently derived from two different directions: "dumb" as "unable to speak", because the mute were considered to be mentally feeble; and "dummy" as "mannequin", which of course wasn't very bright at all, having a wooden head.
The Mental Health and Learning Disability website places the use of "dumb" to mean "mentally infirm" quite a bit earlier, before 1149 CE, in fact.
dumb: with dual meaning of stupid and mute in many Germanic languages.
"Words in Date Order".
Not nearly as much fun as Bugs Bunny.
Regards--
Mary Anne