'antichristianly' originates from English, specifically the word 'antichristian' plus the adverbial suffix '-ly', where the prefix 'anti-' comes from Greek 'anti' meaning 'against' and 'Christian' comes via Latin 'Christianus' from Greek 'Christos' meaning 'anointed'.

'antichristianly' changed from the adjective 'antichristian' (formed in English by combining 'anti-' + 'Christian') and later had the suffix '-ly' added in Modern English to form the adverbial 'antichristianly', yielding the modern English word.

Initially the elements meant 'against' + 'Christian' referring to opposition to Christianity; over time the compound as an adverb came to mean 'in a manner opposed to Christianity'.