Why is a strike-out called a K?

Answer 1

In the old days, the scorer recorded outs by using the last letter of the way the batter made an out. the last letter of "struck" as in struck out, is the only one that is still with us today. Because "S" was already being used to denote a sacrifice.

Answer 2

It's because newspaperman Henry Chadwick, while refining the technique used for filling out a scorecard, needed an abbreviation for strikeout (S was already being used for a sacrifice) He elected to use "K" because it was the last letter in "struck".