Yo

1. exclamation used to indicate a person's present at roll call;

2. exclamation used to call someone's attention;

3. exclamation used to show approval, excitement, etc.;

4. (used sarcastically) exclamation used to indicate that someone has done something stupid or daggy;

5. expression of agreement; yes (although this interjection has a long history, dating back to Middle English times, its current use in Australia stems from 20th C. US usage)