'insidious' originates from Latin, specifically the word 'insidiosus,' where 'insidiae' meant 'ambush.'

'insidiosus' transformed into the French word 'insidieux,' and eventually became the modern English word 'insidious' through Middle English.

Initially, it meant 'lying in wait for,' but over time it evolved into its current meaning of 'proceeding in a gradual, subtle way, but with harmful effects.'