Here's the official version of the pronunciation and meaning of Lagniappe (right off the T-Shirt):
n. Chiefly Southern Louisiana & Mississippi
A small gift given a customer by a merchant at the time of purchase; broadly: something given or obtained gratuitously or by way of good measure.
[Louisiana French, from American Spanish la ņapa,the gift : la, the (from Latin illa, feminine of ille, that, the; see al-1 in Indo-European roots)+ ņapa (variant of yapa, gift, from Quechua, from yapay, to give more).]
"We picked up one excellent word" wrote Mark Twain in Life on the Mississppi (1883), "a word worth travelling to New Orleans to get; a nice limber, expressive, handy word --'lagniappe.'