K stands for strikeout, which was once referred to as being struck. When the letters for scoring were introduced, the first letter wasn't always used if another letter in the word was considered stronger. Also, s might have been in use for a different action such as a single, sacrifice, or stolen base.
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K = strikeout. You might hear an announcer say that a pitcher that struckout 8 batters in a game had 8 Ks.
It could mean a strike out by a called third strike, but that is usually signified by a reverse K.