'knee-high' originates from Old English, specifically the words 'cnēo' meaning 'knee' and 'hēah' meaning 'high'.

'cnēo' and 'hēah' combined in Old English to form 'cnēohēah', which eventually became the modern English word 'knee-high'.

Initially, it meant 'reaching the height of the knee', and this meaning has largely remained the same in modern usage.