'Underbelly' definitions:

Definition of 'underbelly'

From: WordNet
noun
Lower side; "the underbellies of clouds"
noun
The soft belly or underside of an animal's body [syn: underbelly, underbody]
noun
The quality of being weak or unprotected; "the soft underbelly of the Axis"- Winston Churchill

Definition of 'Underbelly'

From: GCIDE
  • Underbelly \Un"der*bel`ly\, n.
  • 1. The lower ventral part of the abdomen of an animal, especially one that walks on four feet. [PJC]
  • 2. The surface of an object closest to the ground, especially of an extended object such as an airplane. [PJC]
  • 3. The weak point of an object vulnerable to attack; used especially in the phrase "the soft underbelly". [PJC]