What does it mean if a fifth grade boy calls you a nerd?

Answer 1

It means just that: you are a nerd.

By definition: Nerd (Noun)

1. A foolish or contemptible person who lacks social skills or is boringly studious

Or

2. An intelligent, single-minded expert in a particular technical discipline or profession.

Seeing how you are the one who asked the question, I would direct you more towards the first definition I listed, definitely not the second.

Answer 2

It could mean that the boy is teasing or trying to be mean, as calling someone a "nerd" is often used as an insult. However, it's important to consider the context and tone in which it was said to determine the intent behind the words.