'cart-horse' originates as a compound of the English words 'cart' and 'horse'. 'Cart' comes ultimately from Old Norse 'kartr' and Latin 'carrum' meaning 'wheeled vehicle', and 'horse' comes from Old English 'hors' meaning 'horse'.

'cart' evolved through Middle English from Old Norse/Late Latin forms and combined with Old English 'hors' in compounds (e.g. Middle English 'cart hors'), eventually forming the modern English compound 'cart-horse'.

Initially it simply meant 'a horse for pulling a cart'; over time the literal meaning has remained, though it has also acquired a figurative sense of 'workhorse' for a person or thing that does heavy, steady work.