This phrase refers to the short life span of a fly, and also to the fact that they die in large numbers if someone decides to kill them. The earliest printed occurrence is found in the newspaper The Atlanta Constitution in May 1902.
[ Examples ]
1. It was flu season and people in our office were dropping like flies. 2. In the beginning, a lot of people signed up for the health program, but after a few months they started dropping like flies. 3. This summer was excruciating. The heat was unbearable and people were dropping like flies.
