Waterproof outer coat or raincoat. The fabric used for a mackintosh was made waterproof by cementing two layers of it together with rubber dissolved in a coal-tar naphtha solution. Macintosh patented his fabric in 1823. The word mackintosh has become a general term for any raincoat. The method was originally invented by the Scottish surgeon James Syme.
While a student of chemistry at Edinburgh, James Syme discovered a method for dissolving rubber (caoutchouc) in a solvent and using it for making a fabric into waterproof laminates. His solvent consisted of rubber dissolved in a coal-tar naphtha solution. Syme submitted his discovery to the editor of the Annals of Philosophy, but for various reasons publication was delayed. During the interim, Charles Macintosh learned of the method, elaborated on it, developed it further, and patented it for commercial purposes. As a result, the Mackintosh raincoat was born and Syme, who took little interest in commercial matters, lost an inestimable fortune.
The name of the man and the raincoat are written different: Macintosh and Mackintosh, respectively.