'antipestilently' originates from Greek and Latin, specifically the prefix 'anti-' (Greek) meaning 'against' and Latin 'pestis'/'pestilentia' meaning 'plague or pestilence', with the adjectival/ adverbial English suffixes '-ent' and '-ly'.

'antipestilent' and related forms appeared in Late/Medieval Latin and Early Modern English as opposition or protection against 'pestilentia'; English formed the adverb by adding '-ly' to produce 'antipestilently'.

Initially it meant 'against pestilence' or 'acting to prevent plague'; over time the basic sense has remained 'in a way that opposes or prevents epidemic disease.'