sour milk, noun phrase
maas sense 1.
1838 F. Owen Diary (1926) 101He asked..whether we wanted food, and at my request sent for the common beverage ‘amas’ or sour milk, a great refreshment in these warm climes.
1980 S. Afr. Digest 16 May 16This porridge is the favourite of the Zulu, Swazi and Xhosa people who traditionally served it with sour milk (amasi) or wild green vegetable stew.

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