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]