One picture holds both uses: the mind has come off its track. Thoughts that normally run in a straight line are jumping about, names come out wrong, and the person cannot steer. What knocked them off the track is what changes — a fever in one use, happiness in the other — but the off-the-track feeling is the same.
The word goes back to Latin dēlīrō, built from dē- 'away from' and līra, the straight furrow a plough cuts in a field. The literal picture is an ox walking out of the furrow, which is remarkably close to what the word still describes.
這個字回到拉丁文 dēlīrō，由 dē-（離開）加上 līra（犁田犁出來的直溝）組成。字面畫面是犁田時走出了那條直溝。這條線索跟今天的意思接得很近：思緒離開了它該走的那條線。
Wiktionary entry 'delirium', etymology tree listing Latin dē-, līra and dēlīrō (packet source_ref kaikki:3f6f69a190036ae4:1180372234)
Two branches
生病時的神智不清
這一族就是核心畫面本身，而且原因是身體上的：高燒、感染、脫水把人撞離了軌道。
noun · C1
A confused state of mind that starts suddenly when someone is very ill, in which they cannot think clearly, do not know where they are or what time it is, and may see or hear things that are not really there.
譫妄；神智不清
興奮到失控
同一個「出軌」的畫面，把原因換成好事。這是教學上的比喻連結：快樂太強，人一樣冷靜不下來。
noun · C1
A state of wild, uncontrolled excitement or joy, strong enough that people can hardly think straight or hold themselves still.
狂喜；瘋狂的興奮