underbelly

underbelly
    n 1: lower side; "the underbellies of clouds"
    2: the soft belly or underside of an animal's body [syn:
       {underbelly}, {underbody}]
    3: the quality of being weak or unprotected; "the soft
       underbelly of the Axis"- Winston Churchill
    

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]
    

56 Moby Thesaurus words for "underbelly":
      abdomen, abomasum, bay window, bed, bedrock, beerbelly, belly,
      bottom, bottom side, breadbasket, breech, buttocks, craw, crop,
      diaphragm, downside, embonpoint, first stomach, fundament, gizzard,
      gullet, gut, hardpan, honeycomb stomach, kishkes, lower side,
      lowest layer, lowest level, manyplies, maw, midriff, nether side,
      nethermost level, omasum, paunch, pot, potbelly, potgut,
      psalterium, pusgut, rennet bag, reticulum, rock bottom, rumen,
      second stomach, spare tire, stomach, substratum, swagbelly,
      third stomach, tum-tum, tummy, underlayer, underneath, underside,
      ventripotence