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.
demented 可以想成「心智離開了駕駛座」。身體還在動——還在講話、還在做事——但沒有人在掌方向盤。所以這個字形容的一直是外面看得到的樣子：聲音、動作、眼神。至於是誰把心智擠下座位，英文用 with 說出來：demented with grief、demented with worry。
From the word’s history
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, "demented"
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