꾀꼬닥 is a sound we use for describing the sound of dying.

 For example, when you are playing with a child, she can shot a gun with her hand, 

if you want to pretend to be dead, you can say "꾀꼬닥" with some actions.

 We use normally 꾀꼬닥 for oral, not for writing ;)

Also, it is very familiar and casual word. 

hope it could be clear answer!