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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The Collaborative International Dictionary of English v.0.48:

Hum \Hum\, interj. [Cf. Hem, interj.]
   Ahem; hem; an inarticulate sound uttered in a pause of speech
   implying doubt and deliberation. --Pope.
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The Collaborative International Dictionary of English v.0.48:

Hum \Hum\, v. t.
   1. To sing with shut mouth; to murmur without articulation;
      to mumble; as, to hum a tune.
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   2. To express satisfaction with by humming.
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   3. To flatter by approving; to cajole; to impose on; to
      humbug. [Colloq. & Low]
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The Collaborative International Dictionary of English v.0.48:

Hum \Hum\, n.
   1. A low monotonous noise, as of bees in flight, of a swiftly
      revolving top, of a wheel, or the like; a drone; a buzz.
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            The shard-borne beetle with his drowsy hums. --Shak.
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   2. Any inarticulate and buzzing sound; as:
      (a) The confused noise of a crowd or of machinery, etc.,
          heard at a distance; as, the hum of industry.
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                But 'midst the crowd, the hum, the shock of men.
                                                  --Byron.
      (b) A buzz or murmur, as of approbation. --Macaulay.
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   3. An imposition or hoax.
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   4. [Cf. Hem, interj.] An inarticulate nasal sound or
      murmur, like h'm, uttered by a speaker in pause from
      embarrassment, affectation, etc.
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            These shrugs, these hums and ha's.    --Shak.
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   5. [Perh. so called because strongly intoxicating.] A kind of
      strong drink formerly used. [Obs.] --Beau. & Fl.
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   Venous hum. See under Venous.
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WordNet (r) 3.0 (2006):

hum
    n 1: the state of being or appearing to be actively engaged in
         an activity; "they manifested all the busyness of a pack of
         beavers"; "there is a constant hum of military preparation"
         [syn: busyness, hum]
    2: an Islamic fundamentalist group in Pakistan that fought the
       Soviet Union in Afghanistan in the 1980s; now operates as a
       terrorist organization primarily in Kashmir and seeks
       Kashmir's accession by Pakistan [syn: Harkat-ul-Mujahidin,
       HUM, Harkat ul-Ansar, HUA, Harkat ul-Mujahedeen, Al
       Faran, Movement of Holy Warriors]
    3: a humming noise; "the hum of distant traffic" [syn: hum,
       humming]
    v 1: sing with closed lips; "She hummed a melody"
    2: be noisy with activity; "This office is buzzing with
       activity" [syn: hum, buzz, seethe]
    3: sound with a monotonous hum [syn: hum, thrum]
    4: make a low continuous sound; "The refrigerator is humming"