English▸ verb [with object] preoccupy or fill the mind of (someone) continually, intrusively, and to a troubling extent:
he was obsessed with the theme of death
(as adjective obsessed) he became completely obsessed about germs.
▪ [no object] (of a person) be preoccupied with or constantly worrying about something:
her husband, who is obsessing about the wrong she has done him.
– ORIGIN late Middle English (in the sense ‘haunt, possess’, referring to an evil spirit): from Latin obsess- ‘besieged’, from the verb obsidere, from ob- ‘opposite’ + sedere ‘sit’. The current sense dates from the late 19th century.