Do you have an ostentatious red-head in your life who has used the phrase "MDK" and your furiously pounded google search leaves you with "michael denmark kuller" or something?
Get the facts here:
MDK is Murder Death Kill and was coined in the hit 1998 movie, Demolition Man.
MDK is a phrase which is used generally and usually as shorthand or meant to be humorous.
The movie in all it's future-seeing glory can be purchased or rented here: https://play.google.com/store/movies/details/Demolition_Man?id=JrfqyPDs82U&hl=en