K-rad was made popular in the 1980s by computer hacker circles. The word 'rad' reffered to a radical act performed by a hacker which deserved congratulations.
The term k-rad originated around the same time people started talking in numbers l1k3 th15. This type of language first started apearing on BBSes and FTP warez sites.
(I have the most leet, k-rad skillz on the net!)