A slang variant of Kilometer, first used by Artillery Division Marines in Vietnam.
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.
"Confirmed NVA contacts, 30 Kliks out, Captain. Bracket those Charlie bastards, and
Bring the Rain."
"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."