yo

UK: /jəʊ/ US: (yō)

WordReference Random House Unabridged Dictionary of American English © 2025

yo  (yō), interj. 

1. (used as an exclamation to get someone's attention, express excitement, greet someone, etc.)
2. here;
   present: used esp. in answer to a roll call.

• late Middle English 1375–1425

y.o., 

1. year old;
   years old.

Collins Concise English Dictionary © HarperCollins Publishers::

yo / jəʊ/ sentence_substitute

1. an expression used as a greeting, to attract someone's attention, etc

Etymology: 20th Century: of unknown origin