short for cyan, magenta, yellow, and key (black)
it is also a webcomic made by Andrew Joseph Koenig. (who is a super duper awesome fellow. all praise to him)
until he gets a site, you can check it out here
guy 1: yo dude, you check out the new CMYK comic yet?
guy 2: no lol, what's it about?
guy 1: roflmaolololol can't explain, just check it.
Cyan, Magenta, Yellow, blacK. The colors used to make color printouts from either inkjet or laser printers.
It sucks that I have a CMYK cartridge, instead of individual ink tanks like Canon has.