The most offensive name-calling

I had a friend on the Internet. We played online games together with him. For a couple, we all bent down with him. There were, however, and epic files. You see, I'm a hot man. I often yell at people. When something didn't stick, I called my friend an eccentric on the letter m, and sent 3 letters, and swore at him how much the light costs. But he never took offense at me: friend, bullshit. But once I jokingly called him a nerd. Suddenly he rage`fired not childishly. I scribbled a multi-page copy-paste with a three-story obscenity in my LAN, retired from my friends, left the clan in an online game and never got in touch again.
Dear experts, answer the question: why is "nerd" the most offensive word?