The term originates from the sights of a Field Artillery gun, which were set to Kilometers, and when changed made a "Klik" sound. Eventually, this onomatopoeia, when combined with Kilometer, was turned into Military slang.
Currently, the term has fallen out of use with Artillery divisions, as their sighting systems no longer make clicking noises, but it has come to be used in almost every US Military service in some fashion or another, and has become a standard military term.
see: Click, and Klick.
"We've got two Kliks to extract point Whiskey. We go double-time, and we might make it in time to catch a ride Stateside. Ooh-Rah."