Using Websters is easier...
Here's Dictionary.com's definiton:
–noun Slang .
1.
a stupid, irritating, ineffectual, or unattractive person.
2.
an intelligent but single-minded person obsessed with a nonsocial hobby or pursuit: a computer nerd.
I would add to that the (presumed) trait of being socially awkward. Seems more apt than the idea of being unattractive, 'cuz I think guys who wear glasses can be very cute and endearing-looking. The non-social description used in the 2nd definition is, I believe, more likely to spring to mind than the word "ineffectual," but hey, that's from Dictionary.com.
Here's Webster's definition:
: an unstylish, unattractive, or socially inept person;
especially : one slavishly devoted to intellectual or academic pursuits <computer nerds>
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