'anti-Trinitarian' originates from Modern English, formed by the prefix 'anti-' (from Greek 'anti', meaning 'against') combined with 'Trinitarian' (from 'Trinity').

'Trinity' comes from Latin 'trinitas' meaning 'threeness'; from that developed 'Trinitarian' (one relating to the Trinity), and from the 17th century onward English combined 'anti-' with 'Trinitarian' to denote opposition, yielding 'anti-Trinitarian'.

Initially it meant 'against the doctrine of the Trinity'; over time the basic meaning has remained the same, denoting opposition to Trinitarian belief or identification of someone who holds that opposition.