'antiecclesiastical' is formed in Modern English from the prefix 'anti-' (from Greek 'anti' meaning 'against') combined with 'ecclesiastical' (meaning 'of or relating to the church').

'ecclesiastical' comes from Late Latin 'ecclesiasticus', from Greek 'ekklesiastikos' (from 'ekklesia' meaning 'assembly, church'); the English adjective 'ecclesiastical' developed via Late Latin and Middle English forms into modern English, and the prefix 'anti-' was attached to form 'antiecclesiastical'.

The elements originally meant 'against' + 'relating to the church'; the compound has retained that meaning as 'opposed to the church or church authority'.