을 만큼 먹다

had my fill; old enough

Repeating the verb around -을 만큼 says 'as much as there was to have': 먹을 만큼 먹었다 is 'I've had my fill', and by extension 'that's enough of that'. Several lines here attach it to 나이 — 나이 먹을 만큼 먹었는데 means 'I'm plenty old enough', said when an adult is being treated like a child. One line applies it to money: 먹을 만큼 먹고 단타 치고 빠지겠다, take your gains and get out.

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