The Collaborative International Dictionary of English v.0.48:

Dim \Dim\, a. [Compar. Dimmer; superl. Dimmest.] [AS. dim;
   akin to OFries. dim, Icel. dimmr: cf. MHG. timmer, timber; of
   uncertain origin.]
   1. Not bright or distinct; wanting luminousness or clearness;
      obscure in luster or sound; dusky; darkish; obscure;
      indistinct; overcast; tarnished.
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            The dim magnificence of poetry.       --Whewell.
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            How is the gold become dim!           --Lam. iv. 1.
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            I never saw
            The heavens so dim by day.            --Shak.
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            Three sleepless nights I passed in sounding on,
            Through words and things, a dim and perilous way.
                                                  --Wordsworth.
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   2. Of obscure vision; not seeing clearly; hence, dull of
      apprehension; of weak perception; obtuse.
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            Mine eye also is dim by reason of sorrow. --Job
                                                  xvii. 7.
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            The understanding is dim.             --Rogers.
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   Note: Obvious compounds: dim-eyed; dim-sighted, etc.

   Syn: Obscure; dusky; dark; mysterious; imperfect; dull;
        sullied; tarnished.
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WordNet (r) 3.0 (2006):

dimmer
    n 1: a rheostat that varies the current through an electric
         light in order to control the level of illumination