I would call the word itself a pet term. This is an interesting topic, and pet terms are probably common within families and other small, tight-knit groups.
An example of this usage, in a headline from a media analysis website:
Limbaugh Explains His Pet Term "New Castrati": Men Who Are "Bullied By Women And The Power Structure And Liberalism"
Note that this is not the same as a term of endearment.
EDIT: This is not an established linguistic term. I consulted a few sociolinguistics textbooks and didn’t find any discussion of this kind of ephemeral in-group language. One place to look would be studies of college slang, like Connie Eble’s Slang and Sociability.
Pet term is also different from pet word, which seems to have a generally accepted meaning: a word that is frequently used or otherwise favored by an individual person or writer. Pet words are already part of the general lexicon, though. Brad Leithauser has a 2013 New Yorker piece about pet words. He gives some examples (sweet for Shakespeare, lad for A.E. Housman) and compares them to stray cats taken in by their users:
Each of these words presents the critic with a little puzzle of devotion: What was it about this particular package of syllables? Why was this stray cat escorted into the author’s studio and offered a saucer of cream and a plump pillow by the fireplace? It’s not as though the studio were soundproof; during working hours, the author no doubt could hear other strays, seemingly no less deserving, meowing clamorously for admission.
Link: Pet Words
So, pet term (rather than word) can convey that it's favored by the in-group, but not established as a true word yet.
I would go with disillusionment.
A feeling of disappointment, akin to depression, arising from the realization that something is not what it was expected or believed to be, possibly accompanied by philosophical angst from having one's beliefs challenged.
[Wiktionary]
An example from vocabulary.com:
Disillusionment is when the hard truth of reality makes you lose faith in your dreams and ideals. Like you might have thought your internship at the movie studio would be super glamorous, but you quickly suffered some disillusionment when you realized it involved 14-hour days and lots of coffee runs.
A deep distinction between disillusionment and disappointment from the book "The Poetics of Disappointment: Wordsworth to Ashbery" By Quinney:
Best Answer
The process of joining groups of humans and things are fairly distinct and probably call for different terms.
For humans, the verb substitute, or sub for short, may suit. The Macmillan Dictionary defines it as
For objects, the verb tack may be suitable. According to Collins, it means
This can be applied to other materials, including metals, although some tacking processes (such as spot welding) may require a good bit of effort to reverse.