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.
From the word’s history
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, "gaga"
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