Certain words you hear in English are only ever heard in a single context. For example, skirl is used to describe the sound a bagpipe makes. Etymonline generously says the word is "rarely" heard outside that context, but I can't recall ever hearing it used for anything else. I imagine one could use it figuratively to describe another godawful high-pitched screech (sorry, bagpipe lovers), but there's no other bona fide usage for it.
What I want to know is stated in the title of the question: Is there a term for these one-off words? I'm sure there must be, but I can't think of what it might be.
Edit: Judging from some of the head-scratching comments I've received, there seems to be some confusion. Perhaps I did not make my meaning clear. I'm not looking for a word to describe the single instance of skirl. I'm asking about a class of words like skirl. I know there exist other examples of words that are only ever used in one context, but I can't think of any others at the moment.