Up till now Tirian had taken it for granted that the horses which the Calormenes were driving were their own horses; dumb, witless animals like the horses of our own world.
For every one of them had been taught — when it was only a chick or a puppy or a cub — how Aslan at the beginning of the world had turned the beasts of Narnia into Talking Beasts and warned them that if they weren’t good they might one day be turned back again and be like the poor witless animals one meets in other countries.
There are no more uses of "witless" in the book.
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They were yammering away in some language Yetta didn’t recognize, or maybe it wasn’t a language at all, just witless jabbering.