'equinumerous' originates from Latin elements: the prefix 'equi-' from Latin 'aequus' meaning 'equal', combined with 'numerous' from Latin 'numerus' meaning 'number' (via Late Latin/Latin-derived formation).

'equinumerous' was formed in modern English by combining the Latin-derived prefix 'equi-' and the adjective 'numerous'; it reflects a compound built from classical roots rather than descending from a single older English word.

Initially and historically the components meant 'equal' and 'number'; the combined word has retained that combined sense of 'equal in number' and has been used in general and mathematical contexts.