Green’s Dictionary of Slang

(US black teen) a general greeting.

1992		Eble Campus Sl. Apr. 10: what up – greeting.
1992		D. Burke Street Talk 2 51: ‘What up toe?’ – ‘How are you?’.
2003		Eminem F/ G-Unit ‘Bump Heads’ 🎵 on Hail Mary/Bump Heads/The Realest Label (White Label) [12"] So what’s the, deal, what’s with all the tough talk / When I walked up to you, like ‘Ja, what up dog?’ / How come you didn’t say, you had a problem then / When you were standing there with all ya men.