Where does “kilo-” come from?

kilo- (Spanish) comes from French kilo-, from Ancient Greek χίλιοι, from Proto-Hellenic *kʰehliyoi, from Proto-Indo-European ǵʰéslom, from Proto-Indo-European sm̥-, from Proto-Indo-European sem- — together, one.

kilo- (Spanish): kilo- SI prefix multiplying the unit by 103