'pestilently' originates from Latin, specifically the adjective 'pestilens', where 'pestis' meant 'plague'.

'pestilens' passed into Medieval Latin and Old French (e.g. 'pestilent') and then into Middle English as 'pestilent' (adjective); the adverbial form 'pestilently' was formed in English by adding the suffix '-ly'.

Initially it meant 'relating to or causing plague', but over time it evolved to mean 'in a destructive, harmful, or noxious manner' and is now used more broadly beyond literal plague.