'underactive' originates from modern English formation combining the Old English prefix 'under' and the adjective 'active' (from Latin 'activus'), where the prefix 'under-' meant 'below' or 'less than' and 'activus' meant 'active'.

'under' comes from Old English 'under'; 'active' comes from Latin 'activus' (from 'actus' related to 'do/act') via Old French and Middle English 'active'; these elements were combined in modern English to form 'underactive'.

Initially a straightforward combination meaning 'below (in) activity', it has come to be used especially in medical and descriptive contexts to mean 'insufficiently active' or 'functioning below normal levels'.