it could be native to the Cape Horn area, since that's where it was first applied, according to the citations.

Hmmm...interesting...I wonder, then, if williwaw could be traced to an indigenous South American tongue?
'Course that particular area, being a timeless seaway, is such a melting pot of multi-national settlers that it could be derived from any one of a wide assortment of languages. Guess we'll never know.