How do you say "taxpayer" in Latin?

The word I am trying to discover is how you would say “thing of the taxpayer”. (Like res publica but referencing the taxpayer rather than the public.) So I suppose I am looking for the genitive form of taxpayer, if that is one word. An online dictionary is a little help.

Since it probably isn’t, I’m not sure if I can make a noun string (“tax”, vectigal, as an adjective?), or if I have to say “thing of the payer(s) of tax(es)”, which probably won’t be quite as snappy as res publica.

Google Translate renders it as adsiduus.

I guess there is a Google for everything! Thanks.

Now I am going to figure out how to put it in the genitive, and figure out if adsidui is plural or a singular referring to a group (like team, I suppose)? And then there is the matter of whether this is a first declension…

Why did I think this was going to be easy?

There is a Wikipedia article for that: