'pestilent' originates from Latin, specifically the word 'pestilentia' (related to 'pestis'), where 'pestis' meant 'plague' or 'pestilence'.

'pestilent' changed from Old French (e.g. 'pestilente') and Middle English 'pestilent', ultimately deriving from Latin 'pestilentia' (from 'pestis'), and became the modern English word 'pestilent'.

Initially, it meant 'causing or relating to plague', but over time it broadened to mean 'causing great harm or annoyance' and 'very harmful or noxious' in general.