Mental exhaustion or fatigue from overwork; a tired or confused mental state.
Compound of 'brain' and 'fag' (British slang for a tiring task or cigarette, from Old English 'fagian' meaning to struggle or tire). The term describes the fatigue that results from taxing the brain.
This Victorian-era term perfectly captures what we now call 'burnout,' but it was already recognized in the 1800s as a real medical concern. Doctors worried about it in their patients—they understood that brains could be genuinely overworked.
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