It's derived from the Greek word "khílioi" which means "thousand".
This Greek root is also found in many other words related to thousands, such as:
* Kilogram: a unit of mass equal to 1,000 grams
* Kilometer: a unit of distance equal to 1,000 meters
* Kilowatt: a unit of power equal to 1,000 watts
The use of "kilo" as a prefix to denote "thousand" became widespread in scientific and technical fields during the 18th and 19th centuries, thanks to the influence of the metric system.