CPDK is an acronym which stands for Chicago Police Department Killer. This acronym is widely used by criminals in Chicago, especially gang members, to signal their rivalry with cops. Anyone claiming they are CPDK mean they see cops as their enemies and have no problem with killing one. Adding a 'K' after certain item in Chicago means that the item in question is an enemy. Gang members are well known for adding K after their rivals' name, for instance 'BDK' and 'GDK' are 2 very common sayings in Chicago.
As Drill Music which originated in Chicago started growing, Chicago slangs which are heavily used by Drill Rappers started becoming popular with those outside of Chicago, including CPDK. In Lil Jay's Bar Of Clout song, he said "CPDK, middle fingers to the law - And if I got a problem with an opp, - I'll have Wooskie knock them off"