'undernourishment' originates from the prefix 'under-' meaning 'below' and the word 'nourishment' from Old French 'nourrissement', which meant 'feeding or nurturing'.

'nourrissement' transformed into the Middle English word 'nourishment', and eventually became the modern English word 'nourishment'. The prefix 'under-' was added to denote insufficiency.

Initially, it meant 'feeding or nurturing below the required level', and this meaning has largely remained the same in modern usage.