Friable, crumbly, or loose rock, typically considered unsafe or unpleasant to climb.
1930s; earliest use found in Margaret Mitchell (1900–1949), novelist. Representing a colloquial or humorous pronunciation of chaos.
"My hands were torn and bleeding already, so I paused to wipe them, one at a time, but I lost my grip and bounced from front to back over the choss, transferred by the tumbling rubble to the bottom of the wall."
- Lori Lansens, The Mountain Story