The Collaborative International Dictionary of English v.0.48:

Fusel \Fu"sel\, n., Fusel oil \Fu"sel oil\ [G. fusel bad
   liquor.] (Chem.)
   A hot, acrid, oily liquid, accompanying many alcoholic
   liquors (as potato whisky, corn whisky, etc.), as an
   undesirable ingredient, and consisting of several of the
   higher alcohols and compound ethers, but mostly of amyl
   alcohol; hence, amyl alcohol or a mixture of amyl alcohols.
   [1913 Webster +PJC]