eleventh Roman letter, from Greek kappa, from Phoenician kaph or a similar Semitic source, said to mean literally "hollow of the hand" and to be so called for its shape.
Little used in classical Latin, which at an early age conformed most of its words (the exceptions had ritual importance) to a spelling using -c- (a character derived from Greek gamma). In Late Latin, pronunciation of -c- shifted (in the direction of "s"). Greek names brought into Latin also were regularized with a -c- spelling, and then underwent the Late Latin sound-shift; hence the modern pronunciation of Cyrus, Circe. To keep their pronunciation clear, the many Greek words (often Church words) that entered Latin after this shift tended to take Latin -k- for Greek kappa.
K- thus became a supplementary letter to -c- in Medieval Latin, used with Greek and foreign words. But most of the languages descended from Latin had little need of it, having evolved other solutions to the sound shifts.
K- also was scarce in Old English. After the Norman conquest, new scribal habits restricted -c- and expanded the use of -k-, which began to be common in English spelling from 13c. This probably was done because the sound value of -c- was evolving in French and the other letter was available to clearly mark the "k" sound for scribes working in English. For more, see C.
In words transliterated from Arabic, Hebrew, Turkish, Japanese, Hawaiian, etc., it represents several different sounds lumped. In modern use some of them are now with kh-; in older borrowings they often followed traditional English spelling and were written with a C- (Corea, Caaba, etc.).
As a symbol for potassium, it represents Latin kalium "potash." In CMYK as a color system for commercial printing it means "black" but seems to stand for key in a specialized printing sense. 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 represent 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]