If someone “wouldn’t a harm a fly,” that means they are mild, gentle, or kind.
Example: Susie felt that she was married to the most wonderful man. He was very polite and respectful. He was the kind of guy that would hold the door open for someone else, even if he had to sit there and wait for half a minute. Also, he was a caring individual, the type of guy who wouldn’t harm a fly.
Origin – Wouldn’t Harm a Fly
There are plenty of people who would have not hesitate when it comes to harming a fly. And by that I mean killing a fly. With a fly-swatter. Really, flies can be quite annoying. They buzz around your head sometimes when you’re busy working on something. Or they’ll land directly on that delicious meal you just made and were about to eat. How irritating!
In addition, flies are known to carry different diseases with them. These diseases can be transmitted to humans and animals, so that’s problematic. Thus, since most people would have no problem putting an end to a fly’s existence, this phrase suggests that someone is so gentle (or nonviolent) that they would not even harm a single, pesky little fly.
Anyways, I don’t know how old this phrase is, but the earliest I could find this expression in writing is in the late 1700s. However, there is a slight difference—it uses the word ‘hurt’ in place of ‘harm.’ This example is seen in The Critical Review, Or, Annals of Literature, from the year 1778:
“The word that, he thinks, is therefore not to be considered as a conjunction, but as an article, or a pronoun: and to prove this, he produces, among many others, the following example: ‘I wish you to believe, that I would not willfully hurt a fly.'”
The expression with ‘harm’ in it is seen in the European Magazine and London Review, from 1802:
“I remember a General whose generous nature would not harm a fly!”
- Ashley was worried that her children would not treat the tiny chihuahua she just bought with the utmost of care. However, I assured her that they wouldn’t even harm a fly, so it would be okay.
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