K Look up K at Dictionary.com
Roman letter, from Greek kappa, ultimately from Phoenician and general Semitic kaph, said to mean literally "hollow of the hand" and to be so called for its shape. For history of its use, see C. As a symbol for potassium, it represents Latin kalium "potash." Slang meaning "one thousand dollars" is 1970s, from kilo-. K as a measure of capacity (especially in computer memory) meaning "one thousand" also is an abbreviation of kilo-.

As an indication of "strikeout" in baseball score-keeping it dates from 1874 and is said to the last letter of struck. The invention of the scorecard symbols is attributed to English-born U.S. newspaperman Henry Chadwick (1824-1908) principally of the old New York "Clipper," who had been writing baseball since 1858, and who explained it thus:
Smith was the first striker, and went out on three strikes, which is recorded by the figure "1" for the first out, and the letter K to indicate how put out, K being the last letter of the word "struck." The letter K is used in this instance as being easier to remember in connection with the word struck than S, the first letter, would be. [Henry Chadwick, "Chadwick's Base Ball Manual," London, 1874]