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..