I have a question regarding the German word Halli-Hallo.
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Halli Hallo is a joyful informal expression to greet someone. The Halli does not have any special meaning. I think it's very rarely used, especially among adults. Children might use it more often.
If you use it with your (German) friends, you will likely get a laugh as a response, because it sounds funny and foreigners are not really expected to know this expression, so this can catch them by surprise :)
You should never use this in a formal environment (and be cautious in a professional one). It is not impolite or offensive, but Halli Hallo is the wrong register of speech. People might take you less seriously if you use this. For instance in English, you wouldn't say "see you later, alligator" in a formal meeting.
In addition to infinitezero's accurate answer, let me add some practical experiences of this expression, which should also be an answer to joki3's comment above.
A few years ago, my German teacher came to my place of work, where we had 1 hour of German instruction in a classroom setting. She would invariably enter the room with a "Halli Hallo", which most of us never had heard at the start. It was explained to us, that it was indeed a very informal, playful greeting and not for formal settings. I do think her use of the phrase was very wise, because it was a trigger for us to forget our daily work and routine for a while and be a bit more playful and open for learning. No surprise, we started to use it amongst ourselves in class and occassionally amongst each other, when not in meetings. e.g. at the coffee machine or meeting in the corridor.
This could be compared to hearing "yoo hoo" from very well acquainted colleagues as a greeting before a meeting or to really get some attention during a meeting. I have heard this kind of thing very very rarely.