K comes from the Greek word kilo which means a thousand. The Greeks would likewise show million as M, short for Mega. So if we stay consistent with the Greek abbreviations, then billion would be shown as a letter G (Giga). Think of your computer expressing bytes of memory as kilobyte, megabyte, or gigabyte.

K, or k, is the eleventh letter of the Latin alphabet, used in the modern English alphabet, the alphabets of other western European languages and others worldwide. Its name in English is kay (pronounced /ˈkeɪ/), plural kays.

k, eleventh letter of the alphabet. It corresponds to the Semitic kaph and the Greek kappa (Κ). It has changed its shape less perhaps than any other letter in the history of the alphabet. The Semitic form may derive from an earlier sign representing a bent hand.