In 1859, New York Clipper sportswriter Henry Chadwick introduced the concept of a “box score” to baseball coverage. The English-born Chadwick used the letter “K” to denote a strikeout, since he’d already used “S” for sacrifice.
“K” is a prominent letter in the word “strike” and the last letter in the word “struck,” as in “struck out.” Today, fans and scorers use the same shorthand, with a forward “K” for a swinging strikeout and a backward “K” for a batter caught looking at his third strike.
Chadwick is also credited with the creation of batting average, earned run average, the assignment of numbers to each defensive position for scorekeeping purposes and many other common abbreviations (HR for home run, etc.) still used today.