The word tip stands for "to insure promptitude."
(The modern tradition of tipping may have begun in England during the sixteenth century. Brass urns bearing the inscription "To Insure Promptitude" were allegedly placed first in coffee houses and later in local pubs. People tipped in advance by putting money in these urns. It should be noted however there are other theories as to its true origins, some going back to Roman times, but this one is my personal favourite.)