'athanasianist' originates from Greek, specifically the name 'Athanásios' (Ἀθανάσιος), where 'athanatos' meant 'immortal'. The English form is built from 'Athanasian' + the agent suffix '-ist'.

'Athanásios' (Greek) passed into Latin as 'Athanasius'; from Late Latin/Medieval Latin the adjective 'Athanasianus' and the English adjective 'Athanasian' developed, and English formed 'athanasianist' by adding the suffix '-ist' to denote an adherent.

Initially it referred to things 'of or relating to Athanasius' (or his teachings); over time it came to mean specifically 'a supporter or adherent of Athanasian doctrine (especially Trinitarian doctrine)'.