The Answer:

According to Total Baseball, The Ultimate Encyclopedia of Baseball, Third Edition, sportswriter Henry Chadwick is credited with inventing, in the 1850s, the baseball scoring system the media and fans still use today. He was the first person to assign letters for each play that could take place on the field and numbers to every defensive player on the field. That way he could record the result of every at bat easily and quickly. Chadwick needed to use the letter "S" for sacrifice, so he decided a strikeout would be recorded as the letter "K," which was the last letter in the word "struck"—a common term for striking out.

Take note that when the letter "K" appears backwards it means the player struck out "looking," or struck out without swinging at the third strike.

—The Editors