One of the mysteries of the English language finally explained.
A kind of fine-grained grey stone (either a hardened shale or a claystone) formerly obtained along the banks of the Ayr and used for hones and whetstones and for polishing stone and metal.
Late 18th century; earliest use found in John Sinclair (1754–1835), agricultural improver, politician, and codifier of ‘useful knowledge’. From Water of Ayr, the name of the Scottish river (now more commonly known simply as the Ayr) at the mouth of which the town of Ayr stands + stone.
Water of Ayr stone/ˌwɔːtə(r) əv ˈɛː ˌstəʊn/
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