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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)
「惚ける」 has other readings with different meanings.
祖父は最近少し惚けてきたようです。
Grandfather seems to be becoming senile lately.
撮った写真が惚けてしまって残念です。
The photo I took became blurred.
惚けたふりをして質問に答えませんでした。
I pretended to be clueless and didn't answer.