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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.
A greeting or said to get someone's attention; hey: Yo, dudes and babes!
[1859+; even though yo and yoho are very old utterances, found by 1420, the recent revival of yo as a primarily black interjection has spawned comment; Ernest Paolino of Philadelphia, indignant because a New York writer had claimed the syllable for New York, recalls it from the 1930s as shortening of walyo; in the WWII Army it was the common form of here! used in responding to roll-calls]