In the history of the Empire, I'm sure the 'TC' for clones came about retroactively to distinguish a period when troopers were exclusively clones. The abbreviations for the other specialties stemmed from this central Korps, so most of them retain the 'T'. So the short answer is TK = Trooper Korps.
Not to draw too close a parallel, but it was already there - Lucas obviously was creating a 'bad guy' army similar to what he learned about from WWII. So when referring to the troopers I think of them in the German term 'Korps' and hence the 'K'. In the Star Wars universe you see a lot of parallels with our world's military history - their units are named like those of the Roman Legion, the term 'stormtrooper' comes from WWII. Armies always form around their infantry, after all.
My personal take on it is that the stormtroopers were the original trooper (obviously, from the clones, but still the main arm of the military) and hence formed the central Corps of the Imperial forces.
Aldin Johnson wrote:Gordon - I get that question a lot.
