If you can provide more context or a sentence where you heard "hoi" being used, I might be able to give you a better answer.
It's possible that:
* You heard a dialectal variation: Some regional dialects in German might use "hoi" as a greeting or interjection.
* It's a loanword from another language: "Hoi" could be a loanword from another language, like Swiss German, where it's a common greeting.
* You're thinking of a different word: It's possible you're thinking of a similar-sounding word, like "hey," which is used in English and sometimes borrowed into German.
Please give me more information so I can help you better!