Why Do We Say "Hello" And "Hi"? - Dictionary.com
https://www.dictionary.com/e/hello/
Hi is equivalent to hello, but it is considered a little bit more informal in tone. In fact, it was recorded a lot earlier than hello. Hi developed from the Middle English hy, similar to hey and ha. Essentially, these are all terms used to call attention—they’re short and easy to say—that evolved into the greetings we use today.
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