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]