Observing an old and curious Navajo taboo, Narbona was not allowed to look at his mother-in-law, nor she at him. It was a custom designed to keep the peace and, apparently, to avoid sexual tension. In fact, many mothers-in-law in Navajo country went so far as to wear little warning bells on their clothing so that a son-in-law would not round a corner and inadvertently find himself staring at her. This was no small thing, especially if he happened to look her in the eye. Even an accidental violation of the mother-in-law taboo might require that the family hire a healer to perform an elaborate — and expensive — nightchant to undo all the harm that had been done.
So writes Hampton Sides in the book Blood and Thunder: The Epic Story of Kit Carson and the Conquest of the American West. (Thanks to Marginal Revolution and improbable researcher Mark Dionne for bringing this to our attention.)