One thought that has sat down in the doorway of your mind and will not move. Whatever else you try to reach — sleep, work, a conversation — you have to step over it first. The word never says the thought is important; it only says it is in the way, and that it has been there too long.
From Latin obsessus, the past participle of obsideō ‘to sit on or in, to remain, to besiege’, made of ob- ‘before, in front of’ + sedeō ‘to sit’. The same family gives possess and session.
來自拉丁文 obsessus，是 obsideō（坐在…上、留著不走、圍城）的完成被動分詞，由 ob-（在前面）加 sedeō（坐）組成。possess、session 也是同一支。這正好對上今天的核心畫面：一個念頭像圍城的隊伍，坐在門口不散。
Wiktionary entry ‘obsess’ (source_ref kaikki:3f6f69a190036ae4:818210556:15328), etymology_text supplied in this evidence packet
Two branches
念頭佔滿整顆腦袋
這一支就是核心畫面本身：那個念頭把別的想法擠掉，時間可以拖上幾個月、幾年。
verb · B2
If something or someone obsesses you, they fill your mind so completely that you think or worry about them all the time and cannot give your attention to anything else. It is used most often in the passive: be obsessed with or by something.
使著迷、佔滿心思
口語：在小事上繞圈
同一個畫面縮小成日常尺寸：擋在門口的變成一件小事，而且通常只擋幾個小時。
verb · B2
To keep thinking or talking about something much more than it needs, usually a small thing that is bothering you.
一直糾結、想太多