A fear older than words

Another one from Stephen King's The Gunslinger. The gunslinger and the boy have encountered a group of horrifying Slow Mutants:

The gunslinger felt an atavistic crawl in his intestines and privates.

Atavism is the "resemblance to grandparents or more remote ancestors rather than to parents; tendency to reproduce the ancestral type in animals or plants," according to the OED. This is a great turn of phrase by King, invoking an ancient fear like that of a Stone Age man when something howled in the night.

The American Heritage Dictionary etymology: From the French atavisme, from the Latin atavus, meaning ancestor: from atta, father + avus, grandfather." The OED says atavus literally meant "a great-grandfather's grandfather."