English jokes look simple, how to understand them?

This sentence 100% is wrong, and the copied book is wrong.

Whether it is used as a double object, as a "stupid" explanation, or as an parenthesis, punctuation should be used. For example, I eat fish every day.

Of course, if you use fish as an adverb, it is grammatical to modify eat or every here. But fish can't be an adverb, can it

Finally, fish, as an adjective, of course modifies every, but it certainly does not mean "idiot".

You can also make an auxiliary judgment according to the level of the book where the joke is located. This complex and biased grammar and semantics is not expected to appear in purely entertaining popular books.

Conclusion: The book is wrong.