First of all, we have to know what "nerd" itself means. It doesn't just signify a bookish or pedantic person, but rather someone who is socially inept or square (try finding an exactly equivalent word for that in Chinese!), perhaps, but not necessarily, because of a consuming commitment to intellectual or technical pursuits....Much more at the link, including the awareness among native Chinese speakers of the nerd/geek distinction (and familiarity with this Venn diagram).
I asked about thirty native speakers of various Sinitic languages and topolects how they would say "nerd" in Chinese. Around half of them flat out said that you cannot say "nerd" in Chinese, but must borrow the English word. Roughly another quarter mentioned shūdāizi ("bookworm"), or variants thereof, while another quarter listed all sorts of colorful terms meaning — more or less — "fool; blockhead; dolt; dunce; dullard; simpleton; numskull"; etc.), none of which are really comparable to "nerd". I'll just list a few of the more interesting Chinese terms of the latter sort...
IN THE COMMENTS: Earnest Prole said:
For a subtler definition of nerd see this Venn diagram.