What part of speech is "so" or "thereabouts" in "X or so?"

Question

Every dictionary I've checked defines "so" as an adverb or conjunction. "Thereabouts" seems only to be defined as an adverb. So I'm wondering what role the words play in constructions such as

He was about twenty or so.

He was about twenty or thereabouts.

This use is exemplified in OED definition 2 for thereabouts:

2. Transferred to time, quantity, quality, degree, etc. Mostly preceded by or.

Definition 33b for so is similar:

Or about that amount or number; or thereabout.

I'm wondering what could explain the justification for considering words used in this construction adverbs. It seems to me like the words are taking the place of nouns or noun-modifiers.

He was about twenty or twenty-one.

He was about twenty or something like that.

Are the words in this construction really adverbs? If so, what are they modifying and how does the justification work? If not, what grammatical role do they play?

Answer 1

According to Dictionary.com, "so" can be a pronoun, and I agree that is the way it seems to be used in the sentence "He was about twenty or so," since it could be considered to be taking the place of a noun. See the related question Is "so" a pronoun?. The question for "so" seems to have no definitive answer.

However, "thereabouts" is not cited as a pronoun in any dictionary I can find. If we assume that dictionaries are correct (such as Dictionary.com), "thereabouts" is functioning as an adverb indicating degree or amount (answering the question "How much?") and modifying the verb "was". In that case, "thereabouts" is not to be included as a part of the prepositional phrase "about twenty". (Otherwise, it would be the object of preposition and thus a pronoun.) Rather, we can rewrite the sentence as follows to make it clearer:

He was about twenty or was thereabouts.

This is actually correct usage. The Collins Dictionary gives this example from The Sun:

Brighton should be thereabouts again this season.