Where did the military term "mess" as in "mess hall" originate?

When it first appeared in English, mess meant a portion of food. This came from the Old French mes, “a dish”, which in modern French is spelt mets. This comes ultimately from the Latin missus, strictly “to put, send” but which could also mean “a course at a meal” (that is, something put on the table).
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Taken from worldwidewords.com

In the military, there are a lot of food fights. Those halls get really messy.
etymology is for losers!

@Corporate_Avenger has got it right. It came into English via Norman French. The portion or ration given to each soldier.