underbelly (n.)youdaoicibaDictYouDict[underbelly 词源字典]
c. 1600, from under + belly (n.). In figurative sense of "most vulnerable part" it is recorded from Churchill's 1942 speech. Sometimes used erroneously or euphemistically in sense of "seamy or sordid part" of anything.[underbelly etymology, underbelly origin, 英语词源]