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).

In late Latin and the early Romance period the unvoiced velar, represented by C, became palatalized before front vowels, and in the 12th century K was reintroduced as a substitute for C to represent the velar before front vowels since C did duty for both the velar and palatal in such cases and confusion was thus liable ...

The letter 'K' stands for Kilo. It is an abbreviation for Kilogram (kg), which is 1,000 grams. It also represents a quantity of 1,000.

The letter 'K' is an old letter, as it came from the Egyptian hieroglyphics. In the Semitic language, it was given the name 'kaph' which translated into 'palm of the hand. ' In those times, the letter faced the other way. When the Greeks adopted it in 800 BC, it became 'kappa' and flipped to the right.