With origins from Halo 2 matchmaking, "Where dey at" was initially a slurred headset calling to teamates of "where are the players of the opposite team at". The term eventually evolved into a taunt statement upon headshot, betrayal, assasination, and sticky merkings.
Opposing Player X: (stands foolishly in coridoor underneath Lockout busy aiming w/sniper)
Me:(jumps down from behind and assasinates w/shotty.Where Dey At?(in direction of dead body waiting to respawn)