Reminded by What is the grammatical function of so in this sentence, something that has always bothered me is that the word "so" can be used as a pronoun:
It looks like rain
No, I don't think so.
(Where "so" refers to the statement about rain "that it looks like rain".)
Definition of 'so' - see items 21, 22, where they say it is a pronoun:
such as has been stated: to be good and stay so.
something that is about or near the persons or things in question, as in number or amount: Of the original twelve, five or so remain.
Rather, I am not bothered that it might function as a pronoun (weird things happen). I am perfectly fine with it being a pronoun and using it...so. But it never seems to be mentioned in a list of pronouns (as much as memory can serve). It is not in the set of canonical pronouns. "Thus" seems to share this use.
So...(clears throat), what is the provenance and history of this usage? Do other languages have a similar use of a word that introduces a deduction as also a pronoun for a sentence? (And are there any other such non-canonical pronouns?)