yo

interjection

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

1. year old; years old.

sentence substitute

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

as a greeting, 1859, but the word is attested as a sailor’s or huntsman’s utterance since early 15c. Modern popularity dates from World War II (when, it is said, it was a common response at roll calls) and seems to have been most intense in Philadelphia.