I got this word in one of those "Customs and Traditions" classes we took in the Navy. It's not a Navy tradition, specifically, but the facilitator brought it up while we were talking about "P's and Q's" and provided the following.
Back in the day, and I mean waaaaaaay back, like in the dark ages and medieval period, hospitality and service jobs were not generally paid positions(unlike in Greece and Rome, where slaves handled these unpleasant tasks). Poor people would hang around an inn, a pub, a trader's shop, a livery, etc, and offer to do certain chores hoping for a monetary appreciation. For instance, children would hang around the doors of the inn waiting for customers and would offer to carry in their luggage in hopes of a small monetary appreciation (the "tip.") ) Poor people would offer to load the trade goods onto a purchaser's wagon, or stall their horses at the livery. Women would deliver the drinks to the customers at the tables in a pub. The owners of these establishments let them hang about because it saved them the trouble of carrying, loading, stalling, and serving, etc, but didn't pay them a wage to do it.
As time passed and the classes became further separated, the riche became accustomed to having these tasks performed for them, so, around the time of the Renaissance, these industries began paying a small token salary to keep the better workers around, but still relied on the custom to provide the "tip."
More time passed, and it is now ingrained in society to provide a tip for the hospitality and service industries, and the laws have compensated by allowing employers to pay them less.
I have no idea as to the source of the facilitator's information, so don't quote me as gospel. But it's an interesting and viable theory.