Etymology 1 
From ㄱ (giyeok) + additional horizontal line representing burst of aspiration.
ㅋ (k, McCune-Reischauer k')
- 키읔 (kieuk), a jamo (letter) of the alphabet of the Korean writing system, hangeul.
Etymology 2 
Due to the fact that the letter k repeated sounds like a person laughing, putting ㅋ in a consecutive manner stands for laughing in the Korean internet media.
- (Internet slang) laughing.