The Collaborative International Dictionary of English v.0.48:

Hum \Hum\ (h[u^]m), v. i. [imp. & p. p. Hummed; p. pr. & vb.
   n. Humming.] [Of imitative origin; cf. G. hummen, D.
   hommelen. [root]15.]
   1. To make a low, prolonged sound, like that of a bee in
      flight; to drone; to murmur; to buzz; as, a top hums. --P.
      Fletcher.
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            Still humming on, their drowsy course they keep.
                                                  --Pope.
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   2. To make a nasal sound, like that of the letter m
      prolonged, without opening the mouth, or articulating; to
      mumble in monotonous undertone; to drone.
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            The cloudy messenger turns me his back,
            And hums.                             --Shak.
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   3. [Cf. Hum, interj.] To make an inarticulate sound, like
      h'm, through the nose in the process of speaking, from
      embarrassment or a affectation; to hem.
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   4. To express satisfaction by a humming noise.
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            Here the spectators hummed.           --Trial of the
                                                  Regicides.
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   Note: Formerly the habit of audiences was to express
         gratification by humming and displeasure by hissing.
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   5. To have the sensation of a humming noise; as, my head
      hums, -- a pathological condition.
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