not hurt a fly : not injure or upset anyone or anything.

She said the arrest was a mistake, that her husband wouldn't hurt a fly.

Usage notes: also used in the forms can't hurt a fly, couldn't hurt a fly, and would never hurt a fly:

He was so gentle he would never hurt a fly.

This website has been posted for teaching English Grammar and Vocabulary through video lessons and example sentences showing how to use these grammar points and words in a sentence with the meaning in the authantic examples.