Which grammatical/linguistic descriptor does not apply to the term, “helter-skelter”?

A merism is the combination of two contrasting words to refer to an entirety, e.g. to say that someone searched everywhere, an expression is that someone "searched high and low". Merisms also figure in a number of familiar English expressions. The phrase "lock, stock, and barrel" originally referred to the most conspicuous parts of a gun and has now come to refer to the whole of anything that has constituent parts.

The term “helter-skelter” falls into the categories of “fossil word”, “irreversible binomial” and “collocation”, but not into that of a merism.

A fossil word is a word that's broadly obsolete but which remains in current use because it is contained within an idiom that is still in use. “Helter-skelter” derives from Middle English “skelten” ("to hasten") a word no longer seen or used out with the idiom, e.g. "spick" in "spick and span" is a fossil word that is rare and archaic outside the collocation.

An “irreversible binomial” is the term applied to phrases like “the birds and the bees” and “wear and tear”, which have meanings beyond those of the constituent words and are inseparable parts of the English lexicon; the former is an idiom, whilst the latter is a collocation. The words in an irreversible binomial belong to the same part of speech, have some semantic relationship, and are usually connected by an and or an or. They are often near-synonyms or antonyms (“cheek by jowl”, good and evil), alliterate (“cash and carry”), or rhyme (“helter skelter”).