The raincoat is named after a Scottish chemist, but it is usually spelled differently from his name. Charles Macintosh had no k. The coat usually has one.
Macintosh, who lived from 1766 to 1843, found a way to make cloth waterproof. He dissolved rubber in coal-tar naphtha and used it to bond two layers of fabric together. He received patent No. 4804 on 17 June 1823. Coats made from the fabric were reportedly on sale in Glasgow by October that year.
They were not pleasant to wear. Country Life lists the early problems: the coats went stiff in the cold, sticky in the heat, and smelled bad. The rubber also leaked through stitching holes and broke down when it touched the natural oils in wool.
Thomas Hancock, whose rubber-processing machine dated from 1819, helped fix the odor and temperature problems. By 1830 the two men had merged their businesses and opened a factory in Manchester.
The coat took its maker's name within a few years. Merriam-Webster dates mackintosh as a word for a raincoat to 1835, and Etymonline gives 1836. Merriam-Webster also uses the word for the rubberized fabric itself, and lists macintosh as a less common spelling.
The extra k crept in as the word became generic. Country Life links that shift to the Great Exhibition of 1851, after which mackintosh became a general term for rainwear. The surname itself comes from Gaelic Mac an toisich, "son of the chief," according to Etymonline. The original company still trades today, now under the spelling Mackintosh.
Britannica sums up where the word ended up: mackintosh is now a general name for any raincoat, not just one made by Macintosh's process.
The coat belongs to a run of 19th-century British clothing named after real men. Two of the best known, the cardigan and the raglan, come from generals of the Crimean War.