What does "yo" really mean? I know it is slang, but what's its origin?

Native speaker’s answer
Rebecca
"Yo" is an informal way to greet someone or an exclamation! There's no root form of the word, and it's unclear when it started exactly, but it became popular in North America during the 1950s and '60s. Ex: Yo, WhatsUp, everybody? Ex: I don't know, yo. Ex: Yo, yo, calm down.