'ecumenist' originates from Modern English, formed from the noun 'ecumenism' plus the agent suffix '-ist', ultimately from Late Latin and Greek roots where 'oikoumene' meant 'the inhabited (known) world'.

'ecumenist' traces back via Late Latin/Medieval Latin 'ecumenicus' and the adjective 'ecumenical' (from Greek 'oikoumenikos'), with English forms like 'ecumenical' and 'ecumenism' appearing before the agent noun 'ecumenist' entered English usage.

Initially the Greek root 'oikoumene' referred to 'the inhabited world' or 'the whole inhabited earth'; over time the sense shifted in Christian usage to matters concerning the whole church and then to the modern meaning of 'promoter of church unity', which 'ecumenist' denotes.