He is a queer fish, I have failed to understand him.
Here, the underlined phrase means-

• He is a queer fish, I have failed to understand him.
- Here, the underlined phrase means- খ) strange person

• an odd/queer fish
- English Meaning: a strange person; someone who is slightly strange or crazy.
- Bangla Meaning: একজন ব্যক্তি যার আচরণ অদ্ভুত বা অস্বাভাবিক বলে মনে হয়।

• Ex. Sentence: He was a queer fish but I couldn't help feeling sorry for him.
- Bangla Meaning: সে ছিল এক অদ্ভুত ব্যক্তি, কিন্তু আমি তার জন্য দুঃখ বোধ না করে পারিনি।

Source:
1. Cambridge Dictionary.
2. Longman Dictionary.