'antiecclesiastically' originates from modern English, formed from the prefix 'anti-' (from Greek 'anti' meaning 'against'), plus 'ecclesiastical' (from Late Latin 'ecclesiasticus', ultimately from Greek 'ekklesia' meaning 'assembly, church'), with the adverbial suffix '-ly'.

'ekklesia' in Greek became Latin 'ecclesia', which produced Late Latin 'ecclesiasticus' and Middle English 'ecclesiastical'; the modern English adjective 'ecclesiastical' combined with the prefix 'anti-' and the suffix '-ly' to produce 'antiecclesiastically'.

Initially it meant 'against the assembly/church' (from the root 'ekklesia'), and over time it has retained that sense as 'in a manner opposed to the church or its clergy' in modern usage.