Source : Webster's Revised Unabridged Dictionary (1913)

Lagniappe \La*gniappe\, Lagnappe \La*gnappe"\, n. [Etym.
   uncertain.]
   In Louisiana, a trifling present given to customers by
   tradesmen; a gratuity.

         Lagniappe . . .is something thrown in, gratis, for good
         measure.                                 --Mark Twain.

Source : WordNet®

lagniappe
     n : a small gift (especially one given by a merchant to a
         customer who makes a purchase)