- the lower, posterior part of an animal's belly
- any vulnerable or unprotected area, region, point, etc.
- The soft belly or underside of an animal's body. Also called underbody.
- The bottom side or part: scraped the car's underbelly going over the curb.
- The vulnerable or weak part or aspect: “What if quantum theory is wrong? This is the soft underbelly of the whole project of quantum gravity” (Lee Smolin).
- The corrupt or immoral part or aspect: “[a novel] that most effectively exposed the decadent underbelly of corporate high tech” (Theodore Roszack).
- (figuratively) The side which is not normally seen
- The area by the canal is the underbelly of town - not something you'd see in the guidebooks