From:
http://edl.byu.edu/essays/Kweb.rtf
Here it is - news to me!
K, the eleventh letter of the English Alphabet, is borrowed from the Greeks, being the same character as the Greek kappa, answering to the Oriental kaph. It represents a close articulation, formed by pressing the root of the tongue against the upper part of the mouth, with a depression of the lower jaw and opening of the teeth. It is usually denominated a guttural, but is more properly a palatal. ...
As a numeral, K stands for 250; and with a stroke over it, thus, K for 250,000.