This is strictly a historical "why is it the letter k rather than say v for vertical" question -- is it the initial letter of something from a specific language? Is it arbitrary?
While we're at it, did "h" originally stand for "horizontal" or was it something else?
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1.There are a lot of words that can be used but we use letter "k". It just to show that the word represents a constant,or in German konstant. So we can get an idea of use of k mostly for a constant. 2.Its is obvious that "h" doesn't stand for horizontal.It can be any thing but one can think it as a coincidence.
These websites essentially say that $h$ and $k$ weren't used that often, and so they were just used because all the other letters were used up. There is also some probable relationship to the German word "konstant." Link http://ask.metafilter.com/260472/Why-is-the-center-of-a-circle-often-given-as-h-k