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:

'K' was chosen to represent Struck, because S was already taken to mean Stolen base. The K and S are both still used today.

'K' is a Strike

The symbol "K" is used because it is made with 3 strokes of the pencil, symbolizing the 3 strikes for the strikeout. Scorekeepers use a backward K (or a KL, or Kc) when a batter strikes out looking/called 3rd strike.

K stands for 'struck'. Back in the old days, scorekeepers used letters for symbols, K is the only one that we still use today.

Fyi, to help keep things clear in record keeping, a backwards k is used if the player struck out looking.

I always thought that K was short for K.O. like knock out.

Back in the day, score keeping was done entirely with letters, no numbers (Double was D, not 2B). When "strikeout" became an official statistic, "K" was the first letter not already being used (S- Single, T- Triple, R-Run, I-Inning, then K) So K became strikeout, and it was one of the few from that era that actually stuck.

We owe the "K" to an early sportswriter named Henry Chadwick. In fact, Chadwick and another writer named M.J. Kelly are largely responsible for the scoring system baseball uses today. Chadwick already had "S" slated for "sacrifice." So a strikeout became a "K", after the last letter of the word "struck." The reason a strikeout isn't a "T" is because "struck" was the preferred term of the day.

The symbol "K" in baseball scorecards represents "Strikeout." Why? Well, baseball pioneer Henry Chadwick, the NY Herald's first baseball editor, first used the symbol in 1868. As far as he was concerned, the letter "K" was the "prominent letter of the word strike," adding that "the letter K in struck is easier to remember, than S."

While the above answer may have some basis in fact, there is another explanation. There are two ways to strike out. (In fact, there are two ways in which a player can strike out, and reach first base safely, if the third strike is not caught in the air by the catcher. How do you score that? I'm not telling. ;-)) A player can strike out swinging, or be called out on strikes. To make the distinction, the letters SOS and KOS are used. Try to shorten it. KS? COS? CS? It is an important statistic to the manager, that wants to know who will let the third strike go by, and who will swing. A "K" merely denotes a generic strikeout.

This might be a myth but I was taught it was a simple way of counting strikes. First strike, line one "|" second strike line two "/", third strike line three "". That makes your K. The reverse or "backwards" K indicates the third strike was a called strike.

I think it might be short for K.O. or knock/strike out. sort of a link to boxing.

A "k" means a strike out but you can be more specific than that. if it is forwards than that means that they struck out swinging if it is backwards it means that they didn't swing at the third strike

See the Related Link to the left for "The Dugout Review of The Joy of Keeping Score."