To think about something constantly and cannot stop thinking about it, often to an unhealthy or excessive degree.
From Latin 'obsidere,' meaning 'to besiege' or 'to sit upon,' combining 'ob-' (against) and 'sedere' (to sit). The sense shifted from being 'besieged' by a thought to dwelling on it excessively.
The word 'obsess' literally means to be 'besieged' by a thought—it's like your mind is under siege, which perfectly captures what obsessive thinking actually feels like!
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