There are several theories which account for the origin of this word. According to one theory, in the old days when people bumped into each other, the first question they asked was “How are you?” This was the standard practice. Since everyone knew what the first question was going to be, they asked it very quickly.
When “How are you?” is said quickly, it sounds like “hiya”. Soon “hiya” began to replace the much more formal “How are you?” Some people thought that “hiya” was too long and reduced it to “hi”. Therefore when you greet someone, “Hi, how are you?” what you are actually doing is merely repeating yourself. You are saying, “How are you? How are you?”