The use of the last letter of Struck instead of the first to denote a strikeout dates back to when Henry Chadwick developed the box score in the late 1850's. Chadwick often used the last letter instead of the first, especially if he considered that letter to be the more prominent one in the word. Chadwick said "the letter K in struck is easier to remember in connection with the word, than S." He also used L for Foul and D for Catch on Bound. Only the K survived into the 20th Century. Source: Paul Dickson's The New Dickson Baseball Dictionary and Alan Schwarz's The Numbers Game. The common view that the K was used because the S was taken (by Sacrifice, Stolen Base, or Single) appears to be erroneous.
Other answers from the community:
A K in baseball is shorthand for a strikeout.
k stands for strikeout looking K stands for strikeout swinging
It is a reference to how it is kept in the scorebook. A K is a strikeout. A backwards K is a strikeout looking.
If you mean in baseball, K is for strikeout, from the letter in the word.
shut out Its a Strikeout SO - Strikeout, SHO - Shutout K - Strikeout, SO - Shut out
It means strikeout looking as opposed to strikeout swinging.
A 'K' is a strikeout where the batter swung and missed at the third strike. A 'k' is a strikeout where the batter took a called third strike.
K represents a strikeout for a pitcher, or by the hitter.