What is "textbook marginalization"?
Here, "textbook" is an adjective that describes a standard definition or demonstration of something. "Marginalization" is a noun that describes treating a person, group, or idea as inferior and insignificant. Therefore, the speaker is saying that the mom is demonstrating "textbook" (meaning "typical") marginalization. He is saying she is "marginalizing" him (meaning treating him as inferior) by using humor to point out a mistake in his "argument" (meaning "reasoning") for why only fifth-grade students can be on the debate team.
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