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.
A person who pretends to have a big penis but is in theory is a 2 incher. They often are grumpy and enjoy pissing people off, and pretend to have higher authority and are superior to many people. This term is primarily used as a insult for internet users in which are complete pricks and you are having a go at there penis size.
stupid majorwoody thinks hes better then me
that majorwoody told me to call him sir