In Dune (2021), a planetologist and imperial judge Dr. Liet-Kynes speaks to Paul:
Dr. Liet-Kynes: Your desert boots are fitted slip-fashion at the ankles.
what does "slip-fashion" mean?
slip-fashion is not a common English term when referring to the fit of boots. I believe this is lingo that the author of Dune came up with. Its meaning is somewhat up to your interpretation. What the author meant by this has been discussed elsewhere, like in this Reddit post.