What bookdealers really mean. A dictionary by Tom Congalton and Dan Gregory (Between the Covers), with additions from other sources
Usually means that all of the pages in the book have been "washed" and sizing material, such as gelatin or glue, has been re-applied. The washing may have been done to remove stains, writing, or acid from the pages. Sizing provides a protective finish and makes flimsy paper stiff.