underbelly

noun, plural un·der·bel·lies.

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.

noun plural -lies

1. the part of an animal’s belly nearest to the ground
2. a vulnerable or unprotected part, aspect, or region

n.c.1600, from under + belly (n.). In figurative sense of “most vulnerable part” it is recorded from Churchill’s 1942 speech. Sometimes used erroneously in sense of “seamy or sordid part” of anything.