history
Attested in Song Yu's Warring States-era 《神女赋》: '精神恍忽，若有所喜' - 'my spirit in a haze, as though transported'. Originally described the dreamlike trance of an encounter with a goddess; modernized to plain mental fog.
usage
Medical and everyday both. Used for exhaustion, grief, shock, illness, or aftermath of bad news. Neutral-to-concerned tone - the speaker notices the person isn't themselves.
