'Trinitarian' originates from Latin, specifically the Late Latin word 'trinitarius', where 'trinitas' (from 'tri-' meaning 'three') referred to 'threefoldness' or 'the Trinity'.

'Trinitarian' developed from Late Latin 'trinitarius' into Medieval Latin and then into Middle English as 'Trinitarian', carrying the sense related to 'trinity' from Latin into English usage.

Initially it meant 'of or pertaining to the Trinity' in theological Latin; over time it came to denote both an adjective describing relation to the Trinity and a noun for someone who affirms that doctrine.