The Collaborative International Dictionary of English v.0.48:

Arrack \Ar"rack\ (?; 277), n. [Ar. araq sweat, juice, spirituous
   liquor, fr. araqa to sweat. Cf. Rack arrack.]
   A name in the East Indies and the Indian islands for all
   ardent spirits. Arrack is often distilled from a fermented
   mixture of rice, molasses, and palm wine of the cocoanut tree
   or the date palm, etc.
   [1913 Webster]

WordNet (r) 3.0 (2006):

arrack
    n 1: any of various strong liquors distilled from the fermented
         sap of toddy palms or from fermented molasses [syn:
         arrack, arak]