You can picture demented as the mind having left the driver's seat. The body is still going — talking, moving, working — but nothing is steering it. That is why the word is always about what can be seen from outside: the voice, the hands, the eyes. Whatever pushed the mind out of the seat can then be named, and English names it with with: demented with grief, demented with worry.
The word goes back to Latin dēmēns, built from dē- (away) and mēns (mind) — literally, mind away from where it should be. The same mēns gives dementia.
這個字可以追到拉丁文 dēmēns，由 dē-（離開）加 mēns（心智）組成，字面上就是「心智不在原位」。同一個 mēns 也給了英文 dementia。
Wiktionary etymology tree supplied in this evidence packet (kaikki:3f6f69a190036ae4): Latin dēmēns < dē- + mēns, via Vulgar Latin dēmentō and English dement + -ed.
Two branches
心智離開座位的人
這一支直接用畫面講人：一個是情緒把心智擠開，一個是疾病慢慢把它拿走。兩者的外顯樣子不同，但英文用了同一個字。
adjective · C1
Behaving in a wild, out-of-control way, as if you cannot think clearly any more — often because a very strong feeling such as grief, worry, or pain has taken you over.
發狂的、失控的
離譜到不像有人想過的東西
同一個畫面從人搬到東西上：不是誰生病了，是這個安排看起來根本沒有人在掌方向盤。
adjective · C1
Used about an idea, a plan, a schedule, or a thing to say that it is so extreme or so strange that it seems to make no sense at all. The source marks this use as informal.
荒唐的、誇張到不行的