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Japanese: 磑 - がい

…It is a general term for threshing mortars and mills for flour, and was powered by a waterwheel. Water-powered usu and grinders appear in documents from the Han dynasty, but the idiom 碾磑 was widely used in the Tang and Song dynasties, and became an archaic word after the Yuan and Ming dynasties. The commentary on the Yoro Code from the early Heian period, 'Ryoshaku', explains the meaning of the word: 'Mizukarausu (water usu) is...'

From [Agricultural Tools]

…Since the Tang Dynasty, wheat has been the main grain. Among grains, wheat flour is particularly necessary for milling into fine powder, so as wheat became more widespread, grinding mortars (also called mortars) also became more widespread. …

*Some of the terminology explanations that mention "磑" are listed below.

Source | Heibonsha World Encyclopedia 2nd Edition | Information

Japanese:

…脱穀用のつき臼と製粉用のすり臼の総称で，動力には水車が使われた。すでに漢代の文献に水を利用した碓，磨の類が現れるが，唐･宋時代に碾磑の成語が広く使用され，元･明以降には古語となった。その語義を説くものとして〈養老令〉の平安初期の注〈令釈〉は〈水碓（みずからうす）也。…