word-forming element meaning "one thousand," introduced in French 1795, when the metric system was officially adopted there; irregularly reduced from Greek khilioi "thousand," from PIE *gheslo- "thousand," source also of Sanskrit sahasra-, Avestan hazanjra "thousand." "It is usually assumed that Lat. mille should be connected too" [Beekes]; see milli-. "In the metric system, kilo- means multiplied, & milli- divided, by 1000" [Fowler].
"measure of weight," late 14c. The quantity necessary to fill a tun or cask of wine, thus identical to tun (q.v.). The spelling difference became firmly established 18c. Ton of bricks in the colloquial figurative sense of what you come down on someone like is from 1884.
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Definitions of kiloton from WordNet
one thousand tons;
a measure of explosive power (of an atomic weapon) equal to that of 1000 tons of TNT;