What does the C represent in CMYK?

The K in CMYK can can stand for either blacK or Key colour.

The "K" in CMYK stands for key since in four-color printing cyan, magenta, and yellow printing plates are carefully keyed or aligned with the key of the black.

Suggesting that the "K" in CMYK comes from the last letter in "black" and was chosen because B already means is incorrect.

Courtesy of Wikipedia;

http://en.wikipedia.org/wiki/CMYK_color_model