While I have seen several explanations I like this one with a bit of mystery to it:
In the nearly 40 years since Kendall’s piece was published, no one seems to have come any closer to a
conclusive account of the phrase’s origins or intended meaning within the
Lone Ranger series. Kendall herself decided she preferred “the mystery to certainty,” and perhaps the word is indeed a bit more fun that way. On a 1969 episode of
The Johnny Carson Show, Carson did a sketch in which he interviewed Jay Silverheels—who portrayed Tonto on the
Lone Ranger TV series—in character, for a new job at NBC. “
I work 30 years as faithful sidekick for kemosabe,” Silverheels tells Carson in the bit. “Hunt, fish, make food, sew clothes, sweep up, stay awake all night, listen for enemies for kemosabe. Risk life for kemosabe. Thirty lousy years.” When Carson wonders why Tonto’s former employer let him go, the once-faithful sidekick replies, “Him find out what kemosabe mean.”