not hurt a fly

not hurt a fly

To harm nothing or no one. Said of one who is particularly gentle, shy, diffident, or timid by nature. My brother is a very sweet, warm-hearted man who wouldn't hurt a fly. How can you suspect him of committing this crime?
See also: fly, hurt, not
Farlex Dictionary of Idioms.

not hurt a fly

Also, not hurt a flea. Not cause harm to anyone, be gentle and mild, as in Paul's the kindest man-he wouldn't hurt a flea, or Bert has a temper but it's all talk; he wouldn't hurt a fly. Both fly and flea are used in the sense of "a small insignificant animal." [Early 1800s]
See also: fly, hurt, not
The American Heritage® Dictionary of Idioms by Christine Ammer.

wouldn't hurt a fly

or

wouldn't harm a fly

If someone wouldn't hurt a fly or wouldn't harm a fly, they are very kind and gentle. She was such a lovely girl, who wouldn't have hurt a fly. He is, he insists, a pacifist, who would not harm a fly.
See also: fly, hurt
Collins COBUILD Idioms Dictionary, 3rd ed.

wouldn't hurt (or harm) a fly

used to emphasize how inoffensive and harmless a person or animal is.
See also: fly, hurt
Farlex Partner Idioms Dictionary
See also:
  • as one man
  • drive (one) out of office
  • force (one) out of office
  • force out of office
  • give (one) (one's) head
  • give head
  • give somebody their head
  • give someone their head
  • cooking for one
  • 1FTR