She's no chemistry major

Since chemistry is generally defined as a difficult major reserved for only the brightest and most studious students, a girl who is "no chemistry major" is likely of average or below average intelligence.

This phrase is most often used to refer to women who perceive themselves to be much smarter than they actually are.

Jimbo: Did you hear Karen trying to argue about the economy last night? She really made herself look like an idiot.
Nelson: Yeah, she's no chemistry major.

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