"Flies"

"He settled in that room, 

Where for about forty years the village old-timer 

Had quarreled with the housekeeper, 

Looked out the window and swatted flies." 

We take this phrase literally, but it is not about insects at all! The old expression "to kill a fly" means to drink, which is where the expression "to be under a fly" came from. Both expressions come from the name of a small (15–20 gram) shot glass that was called a "fly." 

The expression "to go under a fly," "to be under a fly" has a very clear origin. Instead of taverns, which had been simple drinking establishments, Peter I ordered inns to be opened all across Russia, where people would not only drink, but also eat hearty snacks. To attract customers who were not very pleased with the innovation, it was decided to pour the first shot free of charge. 

But, first, the innkeepers were greedy. 

Second, the size of shot glasses at the time was quite enough to feel a light, pleasant intoxication. 

Third, tavern customers got into the habit of drinking the free first shot and then going on their way. 

Therefore, especially for inns, shot glasses with a capacity of 10–15 ml were ordered — about one tablespoon — so that the innkeepers would not go broke on free drinks, and customers would still want another drink, but this time at their own expense. The new shot glasses were immediately nicknamed "flies." 

And tavern patrons began moving from one establishment to another in order to drink as many free first "flies" as possible. Hence the expression "to go under a fly."