High school (and sometimes college) boys who like sports, are generally huge assholes, get really drunk, say things like "damn son!", and are disrespectful to girls.
Typical nardy conversation:
Nardy 1: "Dude, I was so drunk last night!"
Nardy 2: "I know. I fucked insert random girl"
Nardy 1: "Damn son! Who was it?"
Nardy 2: "You know, just some bitch."
Typical girl conversation:
Girl 1: "Hey, I'm having a party tonight."
Girl 2: "Ew, can we not invite the nardies? I'm actually trying to have fun."
Noun: 1. One who could be classified as a nerd, but exhibits the quality of an athlete as well. 2. A jock or an athlete who has subtle and discrete nerd tendencies.