Webster's Revised Unabridged Dictionary · 1913

Lagniappe

Lagniappe , noun

[Etym. uncertain.]

1.
In Louisiana, a trifling present given to customers by tradesmen; a gratuity.
Lagniappe...is something thrown in, gratis, for good measure. — Mark Twain
2.
A tip or gratuity.
3.
Anything obtained gratuitously or unexpectedly.

Also: Lagnappe