means A small sum of money given to someone for service, beyond the listed price, as a reward or token of thanks.

from The exact origin is uncertain. The popular story claims it's an acronym for 'To Insure Promptitude,' supposedly posted on boxes in old coffee houses — but that's a charming fiction; acronymic word-origins almost never predate the 20th century. More likely 'tip' comes from an earlier slang verb meaning 'to give, hand over, or pass along,' which had been kicking around English thieves' and tavern cant since the 1600s — the same root that lets you 'tip someone a wink' or 'tip off' a friend.