User blog comment:Morgan silve/Hollows/@comment-1932702-20101101142558

I'm pretty sure the term "Hollow" refers to the hole, and the fact that Hollows are basically empty shells of what once used to be souls of Humans, Shinigami, etc.

In case of WW, I'm thinking that was Studio Pierrot's way of representing the fact that WW had no knowledge, memory, emotions, etc, i.e., show him to be, literally, Hollow.