In a way that shows extreme or compulsive focus on something, unable to stop thinking about it.
From Latin 'obsessus' (besieged, occupied), combining 'ob-' (against) and 'sedere' (to sit). Originally meant to be literally besieged or sat upon; evolved to mean mentally besieged by a thought.
The word 'obsess' literally means to be 'sat upon' by an idea—imagine an idea just settling on you and refusing to leave! That's exactly what obsessive thinking feels like, which is why the Latin metaphor is so perfect.
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