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).

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.

K was brought into the Latin alphabet with the name ka /kaː/ to differentiate it from C, named ce (pronounced /keː/) and Q, named qu and pronounced /kuː/. In the earliest Latin inscriptions, the letters C, K and Q were all used to represent the sounds /k/ and /ɡ/ (which were not differentiated in writing).