The Collaborative International Dictionary of English v.0.48:

Dull \Dull\, a. [Compar. Duller; superl. Dullest.] [AS. dol
   foolish; akin to gedwelan to err, D. dol mad, dwalen to
   wander, err, G. toll mad, Goth. dwals foolish, stupid, cf.
   Gr. ? turbid, troubled, Skr. dhvr to cause to fall. Cf.
   Dolt, Dwale, Dwell, Fraud.]
   1. Slow of understanding; wanting readiness of apprehension;
      stupid; doltish; blockish. "Dull at classical learning."
      --Thackeray.
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            She is not bred so dull but she can learn. --Shak.
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   2. Slow in action; sluggish; unready; awkward.
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            This people's heart is waxed gross, and their ears
            are dull of hearing.                  --Matt. xiii.
                                                  15.
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            O, help my weak wit and sharpen my dull tongue.
                                                  --Spenser.
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   3. Insensible; unfeeling.
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            Think me not
            So dull a devil to forget the loss
            Of such a matchless wife.             -- Beau. & Fl.
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   4. Not keen in edge or point; lacking sharpness; blunt. "Thy
      scythe is dull." --Herbert.
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   5. Not bright or clear to the eye; wanting in liveliness of
      color or luster; not vivid; obscure; dim; as, a dull fire
      or lamp; a dull red or yellow; a dull mirror.
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   6. Heavy; gross; cloggy; insensible; spiritless; lifeless;
      inert. "The dull earth." --Shak.
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            As turning the logs will make a dull fire burn, so
            changes of study a dull brain.        -- Longfellow.
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   7. Furnishing little delight, spirit, or variety;
      uninteresting; tedious; cheerless; gloomy; melancholy;
      depressing; as, a dull story or sermon; a dull occupation
      or period; hence, cloudy; overcast; as, a dull day.
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            Along life's dullest, dreariest walk. -- Keble.

   Syn: Lifeless; inanimate; dead; stupid; doltish; heavy;
        sluggish; sleepy; drowsy; gross; cheerless; tedious;
        irksome; dismal; dreary; clouded; tarnished; obtuse. See
        Lifeless.
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The Collaborative International Dictionary of English v.0.48:

Duller \Dull"er\, n.
   One who, or that which, dulls.
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The Collaborative International Dictionary of English v.0.48:

Dull \Dull\, v. t. [imp. & p. p. Duller; p. pr. & vb. n.
   Dulling.]
   1. To deprive of sharpness of edge or point. "This . . .
      dulled their swords." --Bacon.
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            Borrowing dulls the edge of husbandry. --Shak.
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   2. To make dull, stupid, or sluggish; to stupefy, as the
      senses, the feelings, the perceptions, and the like.
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            Those [drugs] she has
            Will stupefy and dull the sense a while. --Shak.
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            Use and custom have so dulled our eyes. --Trench.
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   3. To render dim or obscure; to sully; to tarnish. "Dulls the
      mirror." --Bacon.
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   4. To deprive of liveliness or activity; to render heavy; to
      make inert; to depress; to weary; to sadden.
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            Attention of mind . . . wasted or dulled through
            continuance.                          --Hooker.
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