'buttermilk' originates from Old English, specifically the words 'butere' meaning 'butter' and 'milc' meaning 'milk'.

'butere' and 'milc' combined in Old English to form 'butermylc', which eventually became the modern English word 'buttermilk'.

Initially, it referred to the liquid left after butter churning, and this meaning has largely remained the same in modern usage.