'arianistical' originates from Latin, specifically the word 'Arianus', where 'Arius' was the proper name of the 4th-century presbyter whose teachings produced the term 'Arian'.

'arianistical' changed from Medieval Latin 'arianisticus' (or 'arianus' + adjectival suffix) and later entered English via ecclesiastical Latin and Middle English usages, eventually becoming the modern English adjective 'arianistical'.

Initially it meant 'of or pertaining to Arius or his followers', and over time it has retained that specialized theological meaning referring to doctrines denying the full divinity of Christ.