'antinaturally' originates from Greek prefix 'anti-' meaning 'against' combined with English 'natural' (from Latin 'naturalis' from 'natura' meaning 'birth, nature'), plus the adverbial suffix '-ly'.

'antinatural' formed in English by prefixing 'anti-' to 'natural' (which came into English via Old French and Latin 'naturalis' from 'natura'); the adverb 'antinaturally' developed by adding the English adverbial suffix '-ly'.

Initially it meant 'against nature' (literally opposed to natural order); over time it has remained close to that sense and is used to describe actions or appearances that seem unnatural or abnormal.