Hyphy is word used commonly in the Yay' Area (a.k.a Bay Area) in cities such as Oak Town (Oakland) and the 415 (San Francisco). Hyphy is a word that combines the two words hyper and fly (you are high). Hyphy is most commonly when you "go dumb/stupid." This effectively means smoking marijuana or taking other drugs. Otherwise, hyphy reffers to getting especially hyper, crazy, and even dangerous or irrational. It is used in such local songs as "Tell Men when to Go" (by E40 and Keak da Sneak).
"Let's go hyphy after school with some grapes!"
"You seen Jamal last night?""Yeah dog, he was seriously hyphy."