'non-equinumerous' originates from Modern English, combining the negative prefix 'non-' (from Latin 'non' via Old French) with 'equinumerous', where 'equi-' ultimately comes from Latin 'aequus' meaning 'equal' and 'numerus' meant 'number'.

'equinumerous' developed in English in mathematical usage from the combining form 'equi-' + 'numerous' (influenced by Latin 'aequus' + 'numerus'); the productive prefix 'non-' was later attached to form 'non-equinumerous'.

Initially the elements meant 'equal' and 'number'; 'equinumerous' came to mean 'having equal cardinality', and 'non-equinumerous' has the straightforward negative meaning 'not having equal cardinality'.