Here's the breakdown:
* "Mess": In military context, "mess" refers to a group of officers or enlisted men who eat together. This term dates back to the 16th century, and it was likely derived from the French word "més" which means "mixture" or "mess".
* "Hall": This simply refers to a large room or building, particularly one used for communal activities.
So, a "mess hall" is literally a hall where the mess (group of people eating together) gathers.
This term has been used in the military for centuries, and it eventually spread into civilian use, particularly in institutions like schools, camps, and other places where large groups of people eat together.