an idiom literally 'Benkei's crying spot' meaning someone's weak spot or Achilles heel; also refers to the shin (a vulnerable spot).
英語の発音が彼の弁慶の泣き所だ。
Pronunciation is his weak spot in English.
サッカーでの彼の弁慶の泣き所はスピードの遅さだ。
In soccer, his weak point is a lack of speed.
子どもがふざけて私の弁慶の泣き所（すね）を蹴った。
A child jokingly kicked my shin — my 'Benkei's crying spot.'