I assume the word "yo" is a slang term for saying hello in English, but what's its origin?

Rebecca

That's right, "yo" is a slang word to say hello! It originated in the 14th century, England, and it was used as an exclamation to attract attention or greet someone! So there's ​not too much difference to what it's used for today. It could be that it was also used to answer roll call - to say you're present in a class or likewise. Ex: Yo, what's up? Ex: A: Adam! Are you here? B: Yo!