In a classic Mad Magazine cartoon (that I dimly recall), the Lone Ranger and Tonto are surrounded by a horde of hostile Indian warriors. The Lone Ranger says to Tonto “what do we do, now?,” to which Tonto replies, “what you mean ‘we,’ kemosabe?”
As a long article in Slate.com says:
“Even if you’ve never heard or seen a single episode of Fran Striker’s early 20th-century creation The Lone Ranger—begun on the radio [in 1933], continued in books and on television, and … the big screen—the term kemosabe is likely familiar to you.”
There are amusing suggestions. The word “tonto” means “fool” in Spanish*. “Some people have pointed out that kemosabe sounds a lot like the Spanish phrase ‘quien no sabe'” which means idiot, so the two characters are calling each other stupid over the decades. I suppose this could have happened by accident (or by some devious design), but it seems unlikely.
Various people have tried hard to reconcile the word with Native American languages, and even to assign Tonto to a tribe (which Striker never did – but it was 1933 and “Indian” may have been sufficient for an entertainment.)
We’ll never know, since creator Fran Striker didn’t record where he got the word. My bet is that it was entirely made-up. If so, it can only mean what an episode of the TV show claimed – Tonto tells the Lone Ranger that the word “mean trusty scout.” Any later attempts (that means you, 2013 film) to redefine the word must be rejected. You only get one shot to contribute to a venerable creation’s canon.
*Note: Apparently, when Lone Ranger episodes are presented in Spanish, the trusty scout’s companion is named “Toro”, which means “bull.”