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:
- 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?