‘A fish out of water’ means—

A fish out of water:
English Meaning: someone who is uncomfortable in a specific situation.
Bangla Meaning: অস্বস্তিকর অবস্থা / বেকায়দা অবস্থা

Ex. Sentence: I felt like a fish out of water at my new school.
Bangla Meaning: আমি আমার নতুন স্কুলে খুব অস্বস্তিতে ছিলাম।

অন্যদিকে,
Once in a blue moon means- Rarely.
To make an ass of oneself means- to act foolishly.
A hard/tough nut to crack means- A difficult problem.

Source: lecture.