K Look up K at Dictionary.com
Roman letter, from Greek kappa, ultimately from Phoenician and general Semitic kaph, said to be literally "hollow of the hand," so called for its shape. For more on the history of its use, see see C. As a symbol for potassium, it represents Latin kalium "potash." Slang meaning "one thousand dollars" is 1970s, from kilo-. As an indication of "strikeout" in baseball scorekeeping it dates from 1874, said to be from last letter of struck, perhaps because first letter already was being used as abbreviation for sacrifice. The invention of the scorecard symbols is attributed to U.S. newspaperman Henry Chadwick (1824-1908) of the old New York "Clipper."
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]
K as a measure of capacity (especially in computer memory) or number (especially of salary), meaning "one thousand" is an abbreviation of kilo.