The Sup Nod is the sudden up-down movement you make with your head when you see someone and wish to greet them, as if to say "Sup."
You can say "Sup" or "Wassup" or "Hey" as you do it.
You do not necessarily need to know the person to which you are giving the Sup Nod to. It's just a way to say hey.
The Sup Nod can also be used when you catch someone looking at you and you want them to stop.
Some use it as a way to let the shawties know you lookin
Mimi-Yea, she was with Karen. I gave 'em the Sup Nod.
Jamila-That weird boy is staring hard
Shelby-Just give him the Sup Nod
Kevin-Bruh, did you see that shawty over there?! I was tryna give her the Sup Nod.
Ishmael-Ay bruh she's coo