This question already has an answer here:
The phrase "helter skelter" is strange because you don't see the word "helter" or "skelter" used anywhere, and when I searched Merriam-Webster dictionary for either word it links me to the definition of "helter skelter."
Since helter and skelter are separated by either a space or a hyphen whenever it's used, it seems like a "compound word," but compound words are made up of other words, whereas "helter skelter" is made up of two "non-words" (for lack of a better word (so far)).
What do we call this? Is there a term for a word like "helter" that is only a word when it's paired with another, "skelter?"