Collectible book terms

What does “prone to shaking” mean in regard to book descriptions? I’ve not been able to find it on line and didn’t see it here: https://www.abebooks.com/books/rarebooks/collecting-guide/understanding-rare-boo... I found this term in Lawrence Goldstone’s Used and Rare: Travels in the Book World.

Any ideas?

shaken back
The loosening or breaking of the sewing of a book, particularly in edition binding, resulting in gaps appearing between the sections. Brittleness of the glue on the spine, inadequate spine lining, loose sewing, and careless use can, individually or collectively, lead to this condition.

(From Bookbinding and the Conservation of Books: a Dictionary of Descriptive Terminology, by Matt Roberts and Don Etherington: https://cool.culturalheritage.org/don/)

I would have to see what it is in reference to. But I assume what the seller is saying is that the book in question is easily shaken (or made loose) in it's binding. I have no idea if he is saying that his copy is superior because it is not shaken, or trying to excuse the fact that his copy is shaken. this is the type of statement that can mean little or nothing, according who the seller is. If you know that the seller has considererable experience with the book in question, you might trust his judgment, otherwise it may be questionable.

>2 lilithcat: and >3 Bernarrd: Makes sense. In Used and Rare Goldstone and his wife Nancy were in the beginning stages of book collecting and were puzzling over the terms they found in book catalogs.