Is "helter skelter" a word or two words or two non-words? What do we call it? [duplicate]

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."

https://www.merriam-webster.com/dictionary/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?"


"New words formed by fusing together parts of existing words are known as blends. They used to be called portmanteau words..."

Source: https://en.oxforddictionaries.com/explore/new-word-combining-two-other-words

noun: helter-skelter; plural noun: helter-skelters

  1. disorder; confusion.
    "the helter-skelter of a school day"
  2. BRITISH
    a tall spiral slide winding around a tower at a fair.

Origin
late 16th century (as an adverb): a rhyming jingle of unknown origin, perhaps symbolic of running feet or from Middle English skelte ‘hasten.’

Source: Google Dictionary

It is a compound word, and just because Middle English skelte ‘hasten’ is no longer in use... It doesn't mean the compound form can't persevere.

Helter Skelter, without the hyphen, is the title of a book, a song by The Beatles, and a term in French as well.