The weakest, most vulnerable part of an organization, nation, or other body. The most probably source of the expression is a reference to animals like porcupines, which are well protected on their backs but not on their bellies. It was first used figuratively during World War II, when Sir Winston Churchill described the Allied invasion of Italy as an attack on the soft underbelly of the Axis. It later was extended to various vulnerabilities, as in “She was . . . sticking her knife . . . into the soft underbelly of capitalism” (John Campbell Crosby, Snake, 1977).
See also: soft