lagniappe n.

also lagnappe, "dividend, something extra, present or extra item given by a dealer to a customer to encourage patronage," 1849, from New Orleans creole, of unknown origin though much speculated upon. Originally a bit of something given by New Orleans shopkeepers to customers. Said to be from American Spanish la ñapa "the gift." Klein says this is in turn from Quechua yapa "something added, gift."

We picked up one excellent word — a word worth travelling to New Orleans to get; a nice, limber, expressive, handy word — 'lagniappe.' They pronounce it lanny-yap. It is Spanish — so they said. ["Mark Twain," "Life on the Mississippi," 1883]