Green’s Dictionary of Slang

1. yes!

1918		D.G. Rowse Doughboy Dope 29: You are assisted in answering ‘Yo!’ to your name by the fortunate knowledge of where it comes on the company roster.
1921		Dos Passos Three Soldiers 34: ‘Answer “Here” when your name is called. Allan, B. C.’ ‘Yo!’ came a shrill voice from the end of the line.
1961		M. Terry Old Liberty (1962) 202: ‘Hear all that?’ ‘Yo,’ I said.
1978		R. Price Ladies’ Man (1985) 32: ‘Yo.’ A squat kid [...] raised one finger and tilted forward.
1980		Eble Campus Sl. Spring 6: yo – an expression of agreement.
1989		(con. 1982–6) T. Williams Cocaine Kids (1990) 41: Yo. When people say they want rock, most of ’em don’t know what they mean.

2. (US) a general term of address.

1961		(con. 1945) G. Forbes Goodbye to Some (1963) 94: ‘Greetings.’ ‘Yo.’.
1972		D. Jenkins Semi-Tough 261: ‘Yo, Daddy,’ Jimmy Keith hollered.
1984		Smiley Culture ‘Cockney Translation’ 🎵 We bawl out YOW! While cockneys say Oi!
1987		T. Wolfe Bonfire of the Vanities 4: Yo, Gober!
1999		Hip-Hop Connection Aug. 20: Those halcyon days when saying ‘Yo! Get stupid’ was not a sign of mental illness, but actually a b-boy’s polite request for a cup of tea.
2006		C.W. Ford Deuce’s Wild 44: ‘Yo, dog,’ the tattoed man said.
2011		Wu-Tang Clan ‘Black Diamonds’ 🎵 Yo, loose cannon, forty-deuce flicks, two parrots.
2015		S. King Finders Keepers (2016) 213: Yo, homie, what you doing in there?
2023		Rayman & Blau Riker’s 14: ‘Yo, punk white boy’.

3. used to add emphasis to a statement.

1980		Treacherous Three ‘Body Rock’ 🎵 I got’s up the new that will blow your mind, yo.
1993		Dr Dre ‘187’ 🎵 Stay strapped, cause niggas bust caps every day, yo.
1997–2001		Online Sl. Dict. 🌐 yo [...] 3. most recently, used to call attention to something you’ve said, but used after the sentence. (‘Let’s go buy some food, yo.’).
2005		W.D. Myers Autobiog. of My Dead Brother 200: Yo, man, I’m crazy scared.
2006		C-Note ‘What It Iz’ 🎵 What it is, yo, you already know what time it is.
2010		A$AP Mob ‘Black Mane’ 🎵 ‘Cause I don’t see nobody, it’s like these niggaz went ghost, yo.
2012		W.D. Myers Cruisers: A Star is Born 91: Yo! She [the school principal] called us the Cruisers.