Why is the letter K used to denote a thousand? Is that from the metric system?

When the French created the metric system in 1795, they created the prefix "kilo-" out of the Greek word for 1,000, χίλιοι.

Actually it is not French for thousand. The French word for thousand is mille. That's why we have millimetre and millilitre. Kilo simply means 1000. What I don't understand is why the damn Yankees have different spellings for Metre and Litre etc. when they don't even use them! It's obviously very dumb of the Americans not to adopt the metric system, they are officially behind the time, being the ONLY country in the world to still use imperial measurement.

It's short for kilo which means 1000

Its a short term for kilo and is now being used as K..

Yes, "K" starts the prefix kilo...which means 1,000.