'pestilence-defeating' originates from Modern English, specifically the compound of 'pestilence' + the present participle 'defeating', where 'pestilence' ultimately comes from Latin 'pestilentia' meaning 'plague' and 'defeat' derives from Old French forms (see below).

'pestilence' comes from Latin 'pestilentia' → Old French 'pestilence' → Middle English 'pestilence'; 'defeat' comes via Old French ('desfaire'/'defaite' sense) and Anglo-Norman into Middle English as 'defeten/defeten' and later modern English 'defeat'; the compound formation (noun + present participle) is a productive pattern in Modern English.

Individually, 'pestilence' initially meant 'plague' and 'defeat' meant 'to overcome or render powerless'; combined in Modern English they transparently mean 'overcoming or preventing plague/epidemic' and that compositional meaning has remained clear.