Lagniappe means a little something extra. It's a Louisiana French word, derived from American Spanish la ņapa, and originally meant a gift given to a customer by a merchant at the time of a purchase, such as a 13th beignet when buying a dozen.

The term has been traced back to the Quechua word yapa or nyap.

Mark Twain writes about the word in a chapter on New Orleans in Life on the Mississippi (1883). He called it "a word worth traveling to New Orleans to get":

We picked up one excellent word -- a word worth traveling to New Orleans to get; a nice limber, expressive, handy word — 'lagniappe.' They pronounce it lanny-yap. It is Spanish -- so they said. We discovered it at the head of a column of odds and ends in the Picayune, the first day; heard twenty people use it the second; inquired what it meant the third; adopted it and got facility in swinging it the fourth. It has a restricted meaning, but I think the people spread it out a little when they choose. It is the equivalent of the thirteenth roll in a 'baker's dozen.' It is something thrown in, gratis, for good measure. The custom originated in the Spanish quarter of the city. When a child or a servant buys something in a shop — or even the mayor or the governor, for aught I know -- he finishes the operation by saying --
'Give me something for lagniappe.'

The shopman always responds; gives the child a bit of licorice-root, gives the servant a cheap cigar or a spool of thread, gives the governor -- I don't know what he gives the governor; support, likely.

When you are invited to drink, and this does occur now and then in New Orleans -- and you say, 'What, again? -- no, I've had enough;'the other party says, 'But just this one time more -- this is for lagniappe.' When the beau perceives that he is stacking his compliments a trifle too high, and sees by the young lady's countenance that the edifice would have been better with the top compliment left off, he puts his 'I beg pardon -- no harm intended,' into the briefer form of 'Oh, that's for lagniappe.'

If the waiter in the restaurant stumbles and spills a gill of coffee down the back of your neck, he says 'For lagniappe, sah,' and gets you another cup without extra charge.