'obsess' originates from Latin, specifically the word 'obsidere,' where 'ob-' meant 'against' and 'sedere' meant 'to sit.'

'obsidere' transformed into the French word 'obséder,' and eventually became the modern English word 'obsess' through Middle English.

Initially, it meant 'to besiege or sit against,' but over time it evolved into its current meaning of 'to preoccupy or dominate the thoughts of someone.'