From The Collaborative International Dictionary of English v.0.48:

Underbelly \Un"der*bel`ly\, n.
   1. The lower ventral part of the abdomen of an animal,
      especially one that walks on four feet.

   2. The surface of an object closest to the ground, especially
      of an extended object such as an airplane.

   3. The weak point of an object vulnerable to attack; used
      especially in the phrase "the soft underbelly".
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