I stopped him right away and explained to him that calling me a Chinaman is like calling an African American the n-word, a Mexican the w-word, or a white person the h-word.
He aplogized and I let it go because I knew that he meant no harm. He thought that since I am a man from CHina, I could be called a Chinaman. As a matter of fact, most Chinese immigrants have no problem with being called a Chinaman. They thought the same way as my Armenian friend. They didn't know any better.
That's why it's so important for everyone to learn each other's culture because Chinaman, like Oriental, is another derogatory word.