PRONUNCIATION: ln-yp, lnyp
NOUN: Chiefly Southern Louisiana & Mississippi 1. A small gift presented by a storeowner to a customer with the customer's purchase. 2. An extra or unexpected gift or benefit. Also called Regional boot2. See Regional Note at beignet.
ETYMOLOGY: Louisiana French, from American Spanish la ñapa, the gift : la, the (from Latin illa, feminine of ille, that, the; see al-1 in Appendix I) + ñapa (variant of yapa, gift, from Quechua, from yapay, to give more).
REGIONAL NOTE: Lagniappe derives from New World Spanish la ñapa, “the gift,” and ultimately from Quechua yapay, “to give more.” The word came into the rich Creole dialect mixture of New Orleans and there acquired a French spelling. It is still used in the Gulf states, especially southern Louisiana, to denote a little bonus that a friendly shopkeeper might add to a purchase. By extension, it may mean “an extra or unexpected gift or benefit.” American Heritage®
I seem to remember this discussed on a language site, but can't remember which of the several I like it was.