The use of the letter K to represent a strikeout is one of the most elegant and concise practices in baseball—especially for headline writers. It should come as no surprise, then, that the originator of the abbreviation was the forefather of the modern sportswriter, Henry Chadwick.
The British-born, Brooklyn-based writer for the Long Island Star and The New York Clipper in the mid-19th century is credited with the invention of the box score, one of his many contributions to the game that earned him posthumous election into the Hall of Fame. Baseball in America developed before television, radio, or even widespread photography, which made newspaper reports of the game crucial to the spread of the sport's popularity. Chadwick wasn't the first person to record the runs scored per inning, but Baseball Magazine declared one of his 1859 game summaries as "The First Baseball Box Score Ever Published," and he became known as the founder of the modern scoring system. Many of the shorthands he developed over the following decades are still part of the modern baseball lexicon, including the K.
Although these days score cards use lines to indicate base hits, Chadwick used an S for single, a D for double, and so on. When it came to making an out at the plate, Chadwick needed an abbreviation for what was known at the time as having "struck three times" and made an out. Since S was taken, he went with K, the last letter in "struck."