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to grow senile; to be childish with age
to be mentally slow; to be befuddled
to play stupid; to say something stupid (oft. on purpose; esp. as part of a comedic routine)
最近物忘れが多くてぼけてきたかもしれない。
I have been forgetful lately and might be losing my edge.
まだ若いのにぼけたようなことを言うなと叱られた。
I was scolded for saying something absent-minded despite being young.
頭がぼけないよう毎朝計算問題を解いている。
I solve math problems every morning to keep my mind sharp.