underbelly

The underbelly of something is its secret weakness, something about it that's not protected from harm. Once a computer hacker discovers a network's underbelly, they're easily able to hack into it.

Literally, an underbelly is just that: it's the abdomen of an animal, the soft lower belly. Often, that's also the most vulnerable part of an animal's body, since it contains major organs, and it's where the figurative meaning comes from. An alternate informal definition is "a hidden, criminal part of society," like the underbelly of a city where criminals trade stolen goods.

Definitions of underbelly
  1. noun
    the quality of being weak or unprotected
    “"the soft underbelly of the Axis"- Winston Churchill”
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    type of:
    soft spot, weak part, weak spot
    a place of especial vulnerability
  2. noun
    the soft belly or underside of an animal's body
    synonyms: underbody
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    type of:
    abdomen, belly, stomach, venter
    the region of the body of a vertebrate between the thorax and the pelvis
  3. noun
    lower side
    “the underbellies of clouds”
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    type of:
    bottom, underside, undersurface
    the lower side of anything
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