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It has been suggested that "tips" is an acronym for "To Insure Proper Service" (Or "Prompt" instead of "Proper"). This is what's known as folk etymology, or the result of people trying to derive the origin of a word without the proper background of knowledge to work from. A moment's thought will show that, since you give a tip after a meal, not before, this supposed origin is nonsensical. The actual origin is unknown, but one source suggests that it may be a derivation from an old "rogues' cant" where "tip" meant to "touch lightly."

The current usage of "tip," giving money for good service, has been in use for several centuries.

Had to refresh my memory on this by checking in The Dictionary of Misinformation, by Tom Burnam, 1977, Ballantine Books, ISBN 345-25453-8-195, which I recommend to anyone who likes reading about urban legends, common fallacies, and other corrections of misinformation.