You can think of the person at the back of the room who cannot sit still — pulling faces, not really listening, enjoying the laugh. Nothing about them is and nothing about them is stupid; they are simply not being serious. Everything the word does grows out of that figure: what that person is (a goof), what that person does with an afternoon (goofing), and the kind of careless slip that comes from being half-elsewhere (a goof, to goof up). The word stays light wherever it goes, which is why people use it about themselves so easily.
From the word’s history
The supplied source entry says the word is perhaps from dialectal English goff, a foolish clown, and traces it no further with confidence. If that route is right, the silly person is where the word started, and the mistakes are what that person leaves behind.
來源資料寫的是「可能」來自英國方言的 goff（傻呼呼的小丑），再往前就不敢確定。如果這條線成立，可以想成：這個字是從那個小丑開始的，而那些小差錯，是這種人走過留下的痕跡。
Wiktionary entry for goof, supplied in this packet: "Perhaps from dialectal English goff (“foolish clown”), from earlier goffe, in which case further etymology is uncertain."
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