The Collaborative International Dictionary of English v.0.48:

dingdong \ding"dong`\, ding-dong \ding-dong\, n. [See Ding.]
   1. The sound of, or as of, repeated strokes on a metallic
      body, as a bell; a repeated and monotonous sound.
      [1913 Webster]

   2. (Horol.) An attachment to a clock by which the quarter
      hours are struck upon bells of different tones.

   3. a stupid or foolish person; -- used in a deprecatory or
      contemptuous sense. [slang]

   Syn: ding-a-ling, doofus. [PJC]

WordNet (r) 3.0 (2006):

dingdong
    adv 1: heartily or earnestly; "They fell to work dingdong"
    v 1: go `ding dong', like a bell [syn: ding, dong,
         dingdong]

Moby Thesaurus II by Grady Ward, 1.0:

114 Moby Thesaurus words for "dingdong":
   alliterating, alliteration, alliterative, asshole, assiduously,
   assonance, assonant, belabored, boob, booby, change ringing,
   chanting, chime, chiming, chink, chump, clang, clanging, clangor,
   clank, clanking, cliche-ridden, clink, clockwork regularity,
   constancy, daily round, ding, ding-a-ling, dingbat, dinging,
   dingle, dong, donging, drone, earnestly, even pace, even tenor,
   exhaustively, galoot, gong, goof, harping, humdrum, intensely,
   invariability, jangle, jerk, jerk-off, jingle, jingle-jangle,
   jinglejangle, jingling, jog-trot, klutz, knell, knelling, labored,
   monologue, monotone, monotonous, monotonousness, monotony, mutt,
   near rhyme, orderliness, painstakingly, peal, peal ringing,
   pealing, pitter-patter, prize sap, regularity, repeated sounds,
   repetitiousness, repetitiveness, rhyme, rhymed, rhyming, ring,
   ring changes, ringing, sameliness, sameness, sap, saphead, sawney,
   schlemiel, singsong, slant rhyme, smoothness, sound, sound a knell,
   stale repetition, tedious, tedium, thoroughly, ting, ting-a-ling,
   tingle, tingling, tink, tinkle, tinkling, tinnitus, tintinnabulate,
   toll, tolling, treadmill, trot, undeviation, undifferentiation,
   unnecessary repetition, unremittingly, unvariation