Thinking about something repeatedly and constantly, unable to stop worrying about it or focusing on it.
From Latin 'obsidere' meaning to occupy or besiege, combining 'ob-' (against) and 'sedere' (to sit). Literally it meant to sit against something, blocking it. The psychological meaning developed in English.
People with OCD have brains that get 'stuck' on thoughts because their error-detection circuits are overactive—their brains are obsessing to protect them from imagined dangers, which is exhausting and why therapy helps rewire these circuits!
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