To agree with in a positive maner. Mainly proclaimed by children in middle school (usualy 7th graders) up to freshmen in high school who claim to be "gangsters". see definitions of "douchbags", "word up", "true dat", for more detail.
Nicole: "I hate this class."
Lucie: "I know I can't wait until it's over"
Nicole: "WORD UP YO !"