The meaning of the question seems clear to me (I say this as there seems to be some confusion in the comments): get all synonyms of the word in question, and return the ones that are least commonly used.
For a view on frequency, you could use Google's Ngrams viewer, which has a large corpus of book texts.
This requires you to enter a comma-separated list of words or phrases, so the next step is to generate that list, e.g. from the results of an online thesaurus. This is relatively easily achieved if you don't mind getting your hands dirty with a bit of Javascript. :)
e.g.:
https://www.collinsdictionary.com/dictionary/english-thesaurus/unspeakable
Open the developer console (ctrl-shift-i in Chrome and Firefox). Click the "console" tab. This will let you enter some Javascript. Enter this:
$.map($(".moreAnt .orth"), function(val,i){return $(val).html()}).join(",");
This will return a comma-separated list of words, which can then be copied into Ngrams. It looks like Ngrams has a maximum character size, and/or a maximum number of simultaneous searches, so you might want to search the terms 10 at a time, e.g.
- dreadful,shocking,appalling,evil,awful,overwhelming,horrible,unbelievable
monstrous,from hell
- inconceivable,unimaginable,repellent,abysmal,frightful,heinous,odious,indescribable
- loathsome,abominable,ineffable,beyond words,execrable,unutterable,inexpressible,
beyond description,hellacious,too horrible for words
Of course, you could just type them yourself, if you don't want to get too technical.