means A persistent, intrusive thought or fixation that grips the mind and refuses to let go, often beyond one's control.

from From Latin 'obsidere,' to sit down before, to besiege — built from 'ob-' (against, before) and 'sedere' (to sit). The image is military: an army camped outside your walls, going nowhere. Originally an obsession was a hostile spirit lurking around a person from the outside (as opposed to possession, which got in). Over time the besieging force moved indoors, from demons to thoughts that lay siege to the mind itself.