From The Jargon File (version 4.4.7, 29 Dec 2003):
k-
pref.
[rare; poss fr. kilo- prefix] Extremely. Rare among hackers, but quite
common among crackers and warez d00dz in compounds such as k-kool /
K'kool´/, k-rad /K´rad´/, and k-awesome /K´aw`sm/. Also used to intensify
negatives; thus, k-evil, k-lame, k-screwed, and k-annoying. Overuse of this
prefix, or use in more formal or technical contexts, is considered an
indicator of lamer status.