What do we mean by Mackintosh?

In Saki's story "The Open Window,"  the "bearer of the white Mackintosh" is Mr. Sappleton, who plays a very small role in the story. A Mackintosh is a type of raincoat that became popular in the 1820s. It takes its name from its creator, Scottish chemist Charles Macintosh. Notice that there is no "K" in the creator's name. The addition of the letter "K" became standard later. Notice also the difference in the "K" when it comes to referring to the coat, versus the "Macintosh" with no "K" that refers to the famous computer by Apple. 
This raincoat is meant to be waterproof, which was a pretty innovative concept for its time. The fabric that was primarily used for the making of this raincoat would have been doubled in thickness, and then rubber would be dissolved on the fabric with a mix of coal-tar naphtha. 
Back to the story, the Mackintosh worn by Mr. Sappleton is described as "fairly muddy" since he was out hunting. However, the detail of the white Mackintosh is important because, in Vera's false story about the supposed death of the hunters, she mentions that her aunt told her that 

they went out, her husband with his white waterproof coat over his arm, and Ronnie, her youngest brother, singing 'Bertie, why do you bound?' as he always did to tease her, because she said it got on her nerves.

Remember that, in Vera's story, the men supposedly never returned, but the aunt was presumably waiting for them to return at any time. It is after she tells the story that the men come back from their trip, Framton identifies the coat, and believes it is the ghost of Mrs. Sappleton's husband returning from the dead. It is here when he runs out of the house and is never seen again.