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 particularly of amyl alcohol; hence, specifically applied to amyl alcohol.

Source: Webster's Revised Unabridged Dictionary (1913)