/un"deuhr bel'ee/ , n. , pl. underbellies .
1. the lower abdomen; posterior ventral area, as of an animal's body.
2. the lower surface of an object; underside: the underbelly of an airplane.
3. a vulnerable area; weak point: an attack on the soft underbelly of Europe.
4. a dark, seamy, often hidden area or side: a police officer continually exposed to the underbelly of society.
[ 1600-10; UNDER- + BELLY ]