K

Used as a suffix in gang terminology meaning "Killer", indicating that the speaker(s) are rivals/enemies of whatever was said before the K.

GDK = Gangster Disciples Killer, used by BD/Black Disciples
BDK = Black Disciples Killer, used by GD/Gangster Disciples
Snitch K = Snitch Killer, someone who kills snitches
CK = Crip Killer, used by Bloods
BK* = Blood Killer, used by Crips

*Also can mean Burger King*