a gymnastic apparatus consisting of two wooden or metal balls connected by a short bar serving as a handle, used as a weight for exercising.
a stupid person.

1705–15; dumb + bell1

2. dunce, ignoramus, fool, blockhead, dimwit.
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World English Dictionary
dumbbell (ˈdʌmˌbɛl)
1.  gymnastics, weightlifting an exercising weight consisting of a single bar with a heavy ball or disc at either end
2.  a small wooden object shaped like this used in dog training for the dog to retrieve
3.  slang chiefly (US), (Canadian) a fool

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
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Word Origin & History

"weighted bar used for exercise," 1711, originally an apparatus like that used to ring a church bell, but without the bell; used for exercise but sometimes also to practice ringing changes. Figurative sense of "blockhead" first recorded 1920, Amer.Eng.
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
He says that with each vibration, the dumbbell catches on the spacetime fabric
  and pushes itself up a tiny bit.
To increase difficulty, hold a dumbbell or free-weight plate.
Not the heavy, raging body-building type, but dumbbell free weight lifting.
Anyone who can hoist a dumbbell and print a business card on a home computer
  can call him or herself a personal trainer.
Image for dumbbell
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