'pestilential' originates from Latin, specifically the word 'pestilentialis', where 'pestis' meant 'plague' or 'pestilence'.

'pestilential' changed from Latin 'pestilentialis' (and Old French forms such as 'pestilentiel') and eventually became the modern English word 'pestilential' through Middle English.

Initially, it meant 'relating to or causing pestilence (plague or epidemic)', and over time it has kept that literal sense while also developing figurative senses of 'extremely harmful or pernicious'.