'antipestilence' originates from a combination of the Greek prefix 'anti-' (from Greek 'anti-' meaning 'against') and the word 'pestilence' (from Old French 'pestilence', ultimately from Latin 'pestilentia'/'pestis'), where 'pestis' meant 'plague'.

'antipestilence' was formed in English by combining the prefix 'anti-' with the existing Middle English/Old French word 'pestilence' (Old French 'pestilence' < Latin 'pestilentia'/'pestis'), and so became the modern English compound 'antipestilence'.

Initially it meant 'a thing against plague' (a countermeasure or remedy for pestilence); over time it retained that core sense but became rare and is now chiefly archaic or literary, sometimes broadened to mean any strong preventive against epidemic disease.