What is the original meaning of the tip?

In America alone, tipping is now a $16 billion-a-year industry. Why should tips exist? The conventional wisdom is that tips both reward the efforts of good service and reduces uncomfortable feelings of inequality.
     Such explanations no doubt explain the purported origin of tipping--in the 16th century boxes in English taverns carried the phrase "To Insure Promptitude" (later TIP). But according to new research from Cornell University, tipping no longer serves any useful function.
     Tipping is better explained by culture than by economics. In America, the custom has become institutionalized: it’s regarded as part of the accepted cost of a service. In New York restaurants, failing to tip 15% could mean abuse from the waiter. In Europe, tipping is less common; in many restaurants, tipping is being replaced by a standard service charge. In many Asian countries, tipping has never really caught on.
     How to account for these national differences? Look no further than psychology. People more extrovert, sociable or neurotic tend to tip more. Tipping relieves anxiety about being served by strangers. In America, where people are outgoing and expressive, tipping is about social approval. If you tip badly, people think less of you. Tipping well is a chance to show off. Icelanders by contrast, do not usually tip--a measure of their introversion.