The Collaborative International Dictionary of English v.0.48:

dumbbell \dumb"bell`\, dumb-bell \dumb"-bell`\, n.
   An exercising weight, consisting of two spheres or spheroids,
   connected by a short bar for a handle; used (often in pairs)
   for gymnastic exercise.
   [1913 Webster]

   2. an ignorant or foolish person.

   Syn: dummy, dope, boob, booby, pinhead.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

dumbbell
    n 1: an exercising weight; two spheres connected by a short bar
         that serves as a handle
    2: an ignorant or foolish person [syn: dumbbell, dummy,
       dope, boob, booby, pinhead]

Moby Thesaurus II by Grady Ward, 1.0:

47 Moby Thesaurus words for "dumbbell":
   Boeotian, Indian club, barbell, block, clod, dimwit, dolt, donkey,
   dope, dullard, dullhead, dumb cluck, dummkopf, dummy, dunce,
   exerciser, gowk, horse, idiot, ignoramus, jobbernowl, lackwit,
   lamebrain, lightweight, long horse, looby, loon, moron, niais,
   nincompoop, ninny, ninnyhammer, nitwit, noddy, parallel bars,
   punching bag, put, rings, rowing machine, side horse, simpleton,
   stupid, thickwit, trampoline, trapeze, weight, witling