underbelly
noun [ S ] literary uk/ˈʌn.dəˌbel.i/ us/ˈʌn.dɚˌbel.i/
最薄弱部分，软肋
the weakest or most unpleasant part of something
a film exposing the sordid underbelly of modern urban society 一部揭露现代都市丑恶的阴暗面的电影
Small businesses are the soft underbelly (= weakest parts) of the economy, and they need as much government support as possible. 小企业是经济中最薄弱的环节，它们需要政府尽可能多的支持。