Origin
Where this phrase originated from exactly is unknown. Flies are an insect, and like most insects, their life is fragile, their lifespans are short, and they come in extraordinary numbers. Regardless of what century you lived in, I'm sure flies were just as annoying back then as they are today. They buzz around in your house, often landing in places they are not wanted, forcing your hand to take action. Driven to the point of madness, you begin to hunt down these annoying pests. One by one they fall, killing several of them in minutes.
The phrase may have originated from a scenario like this, right? ANYWAYS:
What is known, is that earliest known use of the saying was used in The Atlanta Constitution newspaper on May 1902, that reads:
"I saw men and women rushing back and forth within the flames. They would run along, then came the choking smoke and they would drop like dead flies."
There's also a story called The Brave Little Tailor, by Brother Grimm, and it features a boy who kills a large number of flies with his belt. Some say the phrase originated from this story.
Examples of The Phrase Being Used
"Our soldiers are dropping like flies out there, we have to retreat immediately!"
"The entire town was infected by the flu, dropping the population that lived within like flies."
"I was playing an online video game with my friends, competing against others from around the world. Needless to say, my team was dropping like flies, leaving me to fend for myself."
Note: For most old phrases, sayings, and idioms, finding the exact person or date in which a phrase was originated is impossible. What's provided is the earliest known date the phrase or saying was being used in.
Usually the phrase is a quote from an old newspaper, book, poem, play, etc. If it is already being used by a newspaper though, then the saying must already be common knowledge among the people of that time. This means the origin of the saying is older.
For instance, if someone or something from 1660 is being quoted using a particular phrase or saying, you can assume that the saying must be older, as it's already popular and being commonly used in 1660. How far back varies from phrase to phrase.
There are some cases though where determining the origin from an exact person is possible, however these are not common. In short, most of the origins for these sayings give you a date so you can get a rough idea on when these phrases were being used.
