Calling someone a dumbbell is to call them stupid or foolish. What is that shape - dumb bell?
early 18th century: originally denoting an apparatus similar to that used to ring a church bell (but without the bell, so noiseless or ‘dumb’);
dumbbell (dating from the 1920s) is an extended use by association with dumb ‘stupid’.
Dumb has meant stupid for centuries in Germanic languages, so it took Americans a little while to catch up.
The supposed synonyms today are "dunce, ignoramus, fool, blockhead, dimwit" but that isn't current language.
Here's an article with current expressions for this:
A Few Cards Short Of A Full Deck
Not The Sharpest Tool In The Shed!
As Smart As Bait
The Light Is On But Nobody’s Home
A Few Beers Short Of A Six-pack
There’s A Village Missing Its Idiot
One Fry Short Of A Happy Meal
Couple Of Quarters Short Of A Roll
The Elevator Doesn’t Go To The Top Floor
Now that sounds more like it. | 
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