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.
      [1913 Webster]

            Still humming on, their drowsy course they keep.
                                                  --Pope.
      [1913 Webster]

   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.
      [1913 Webster]

            The cloudy messenger turns me his back,
            And hums.                             --Shak.
      [1913 Webster]

   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.
      [1913 Webster]

   4. To express satisfaction by a humming noise.
      [1913 Webster]

            Here the spectators hummed.           --Trial of the
                                                  Regicides.
      [1913 Webster]

   Note: Formerly the habit of audiences was to express
         gratification by humming and displeasure by hissing.
         [1913 Webster]

   5. To have the sensation of a humming noise; as, my head
      hums, -- a pathological condition.
      [1913 Webster]

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.
   [1913 Webster]

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.
      [1913 Webster]

   2. To express satisfaction with by humming.
      [1913 Webster]

   3. To flatter by approving; to cajole; to impose on; to
      humbug. [Colloq. & Low]
      [1913 Webster]

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.
      [1913 Webster]

            The shard-borne beetle with his drowsy hums. --Shak.
      [1913 Webster]

   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.
          [1913 Webster]

                But 'midst the crowd, the hum, the shock of men.
                                                  --Byron.
      (b) A buzz or murmur, as of approbation. --Macaulay.
          [1913 Webster]

   3. An imposition or hoax.
      [1913 Webster]

   4. [Cf. Hem, interj.] An inarticulate nasal sound or
      murmur, like h'm, uttered by a speaker in pause from
      embarrassment, affectation, etc.
      [1913 Webster]

            These shrugs, these hums and ha's.    --Shak.
      [1913 Webster]

   5. [Perh. so called because strongly intoxicating.] A kind of
      strong drink formerly used. [Obs.] --Beau. & Fl.
      [1913 Webster]

   Venous hum. See under Venous.
      [1913 Webster]

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"

Moby Thesaurus II by Grady Ward, 1.0:

170 Moby Thesaurus words for "hum":
   Indian file, anthem, array, articulation, ballad, bank, bel canto,
   birdies, birr, blooping, blurping, bombilate, bombilation,
   bombinate, bombination, boom, booming, bravura, bum, burr, buzz,
   buzzing, carol, catena, catenation, chain, chain reaction,
   chaining, chant, chirp, chirrup, choir, choral singing, chorus,
   coloratura, concatenation, connection, consecution,
   continue the same, continuum, course, croon, crooning, cycle,
   descant, descent, distortion, do-re-mi, drag along, drag on, drone,
   droning, endless belt, endless round, falter, feedback, file,
   filiation, flutter, fluttering, folk singing, gamut, gradation,
   halt, haw, hem, hem and haw, hesitate, hissing, howling,
   hum and haw, humming, hymn, intonate, intonation, intone, lilt,
   line, lineage, low rumbling, lyricism, mammer, minstrel, monotone,
   motorboating, nexus, operatic singing, pendulum, periodicity,
   persist, pipe, plenum, powder train, prevail, progression, psalm,
   purr, purring, quaver, queue, range, rank, recurrence,
   reticulation, rotation, roulade, round, routine, row, rumble, run,
   run through, scale, scat, scat singing, scratching, sequence,
   serenade, series, shake, shredding, sing, sing in chorus, singing,
   single file, sol-fa, sol-fa exercise, solfeggio, solmizate,
   solmization, song, spectrum, squeals, stammer, static, string,
   stumble, stutter, succession, swath, thread, thrum, thrumming,
   tier, tonic sol-fa, train, tremolo, trill, troll, tweedle,
   tweedledee, twit, twitter, vocal music, vocalization, vocalize,
   warble, warbling, whir, whirring, whistle, whistles, whiz,
   whizzing, windrow, woomping, wow, wowwows, yodel, yodeling