Someone who has no idea what the possible outcomes of an input will bring about, it relates to the risk one takes when mixing unknown chemicals together, and that a monkey is likely to attempt something multiple times until it gets some kind of response.
If I was you I would not let that guy work on my computer, he is like a Monkey with a chemistryset.
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.
C'mon guys it's not that bad! Learning the chemical composition of organic molecules is interesting. I mean, who wouldn't want to draw the structural formula of a molecule given its IUPAC name?? Okay maybe i'm just a nerd but still...
Nobody:
Me: OMGYASS! Organic chemistry, my favorite class of the day!
Classmate: Ur weird bro