What is implied when one person calls another nerdy?

When one person calls another nerdy, it is implied that the other person has little or no friends, is very smart, likes science fiction or comic books or has hobbies that involve little physical activity but may require a lot of thought. Although this name is sometimes meant as an insult, most of these are compliments or neutral and the physical actions a bully performs on the victim are much worse than their attempted insult.