M.U.S.T.Y. is a mnemonic acronym coined by the American Library Association in cooperation with the Texas State Library back in 1976 to help School librarians "Weed" their collections. The five letters stand for: Misleading, Ugly, Superseded, Trivial, Your collection.
"Misleading" can occur more rapidly in technology than mythology. Look for “dated” popular fiction, obsolete information, books containing racial, cultural or sexual stereotyping.
"Ugly" refers to the physical condition of the book: antiquated appearance, worn-out, frayed, dirty, unable to mend.
"Superseded": there may be newer copies available, duplicate copies, almanacs, yearbooks, encyclopedias superseded by newer editions.
"Trivial": look for appropriateness for the collection. Check for poor writing, inaccurate information, an inappropriate interest or reading level for students.
"Your collection" has no use for the book. It is irrelevant to your curriculum.
"Weeding", BTW, is defined (here) as "the removing of materials from a library collection in a systematic and deliberate way. It is an ongoing part of collection development, a planned and thoughtful action that will ensure library materials are current and enticing." Librarians are encouraged to develop a weeding policy: "This weeding policy should include a justification, rationale, a plan for teacher evaluation of materials being considered for discard and a process for disposal."
All of this seem innocuous enough, until one person decides to "weed" what another person wants retained. And since the "suggested" copyright indicator to consider weeding fiction is "10 years" then it is not just Enid Blyton, Captain W. E. Johns and Hergé who are on the chopping block. Still, you would expect a small school library to have a policy of renewal and you would hope that they would keep up a supply of recent fiction that is of interest to their students.
The reason why I mention the 1976 "MUSTY" system here, now, is that—according to people within State and University libraries who contacted me privately after my post about Monash University Library disposals—the "MUSTY" system is alive and well, it is still being promulgated and versions of it are being imposed in research and reference libraries today. In such libraries, "MUSTY"-systems are a disaster.
The reasons for this are obvious, "dated popular fiction, obsolete information, books containing racial, cultural or sexual stereotyping" can only become the subject of study and analysis if they are available for study.
The same applies to "superseded" editions and "duplicate copies." Recently, I wanted to see how various editors had handled a textual crux in Romeo and Juliet, so I set out to survey the major Shakespeare editions of the twentieth century. You'd think that this would be easy, but with even research libraries individually disposing of "superseded" editions and combining to dispose of "duplicate" copies, I found this very difficult to do and with a number of editions, not possible at all.
As for books with an "antiquated appearance"—which are "worn-out, frayed, dirty"—my suggestion is repair them or replace them! (There is no suggestion in this "MUSTY"-system that books which are "weeded" should be replaced. It is a disposal system designed to make space for other books, not replacement copies.)
And now I must return to Course Book hell.