hi guys! please tell me about informal idiom ?

Idioms form a part of all forms of expression, both formal and informal.

But some idioms are best suited to formal contexts, whilst others are best for informal ones.

The use of an idiom outside its customary level of formality is often an indicator either of the author's degree of respect for formality ('correctness'), or his level of literacy. Well-bred people use idioms in their appropriate circumstances; others may deliberately mis-use idioms to strengthen the expression of their personal prejudices.