The Collaborative International Dictionary of English v.0.48:

Mad-apple \Mad"-ap`ple\, Mad apple \Mad" ap`ple\, n.
   1. (Bot.) The eggplant bush (Solanum melongena). See
      Eggplant.

   Syn: eggplant, aubergine, brinjal, eggplant bush, garden egg,
        Solanum melongena.
        [1913 Webster + WordNet 1.5]

   2. The fruit of the eggplant bush, a large egg-shaped
      vegetable having a shiny skin typically dark purple but
      occasionally white or yellow.

   Syn: eggplant, aubergine.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

mad apple
    n 1: hairy upright herb native to southeastern Asia but widely
         cultivated for its large glossy edible fruit commonly used
         as a vegetable [syn: eggplant, aubergine, brinjal,
         eggplant bush, garden egg, mad apple, Solanum
         melongena]
    2: egg-shaped vegetable having a shiny skin typically dark
       purple but occasionally white or yellow [syn: eggplant,
       aubergine, mad apple]