'obsess' originates from Latin, specifically the word 'obsidēre' (or 'obsidere'), where 'ob-' meant 'against' or 'toward' and 'sedēre' meant 'to sit' (literally 'to besiege, to sit against').

'obsess' changed from Late Latin/Medieval Latin forms such as 'obsessus' and passed into Middle French (e.g. 'obséder') before becoming the modern English 'obsess' via Middle English influence.

Initially, it had the sense of 'to besiege' or 'to occupy (a place)', but over time it evolved into the psychological sense of 'to occupy the mind' and now commonly means 'to preoccupy or trouble the thoughts'.