The Collaborative International Dictionary of English v.0.48:

Backsheesh \Back"sheesh`\, Backshish \Back"shish`\, n. [Pers.
   bakhsh[imac]sh, fr. bakhsh[imac]dan to give.]
   In Egypt and the Turkish empire, a relatively small amount of
   money given for services rendered (as by a waiter); a
   gratuity; a "tip".

   Syn: gratuity, tip, baksheesh, bakshish, bakshis.
        [1913 Webster + WordNet 1.5]

WordNet (r) 3.0 (2006):

backsheesh
    n 1: a relatively small amount of money given for services
         rendered (as by a waiter) [syn: gratuity, tip,
         pourboire, baksheesh, bakshish, bakshis,
         backsheesh]