One way to picture gaga: a mind that has slipped off its usual track. The person is still here and still talking, but the ordinary steering has come loose — sometimes because age has it, sometimes because a feeling or a long hard day has taken the wheel. Whatever the cause, what other people see is the same thing: this person is not running on normal at the moment. The word is casual and in all its uses, which is exactly what makes it sting when it is aimed at someone's grandmother.
The packet's etymology records only that English borrowed gaga from French gaga. No earlier root is given there, so no further story is claimed here.
詞源資料只寫了一件事：英文的 gaga 是從法文 gaga 借來的。更早的來源沒有記載，所以這裡不往下推。上面的核心畫面是一種幫助記憶的想法，不是歷史來源的說明。
Wiktionary etymology field supplied in the evidence packet: Borrowed from French gaga.
Two branches
腦筋不在正常軌道上
這一支最貼近核心畫面：方向盤鬆掉的是思考本身。差別只在鬆掉的原因，一個是年紀，一個是一時的累或太興奮。
adjective · B2
Confused and forgetting things because of old age. This is an unkind word: it treats an old person's confusion as something funny, so it hurts when it is used about a real person.
老糊塗
被喜歡沖昏頭
一樣是方向盤被接管，只是這次接管的是一個人。腦子還很清楚，只是清楚地只裝得下那一個人，所以旁邊的人照樣覺得「他現在不太正常」。
adjective · B2
Feeling a strong, excited love for someone, in a way that shows and that often does not last long. It is followed by about or over.
迷得神魂顛倒