Arpinas

[Lavrentivs]

Bailey writes that Arpinas, -atis, is "an adjectival form from Arpinum, corresponding to Tusculanum from Tusculum." But Arpinas looks feminine and substantive to me -- ?

[Sceptra Tenens]

That's a Greek form; ας is a masculine termination.

But an adjective? That seems odd to me, since I don't remember ever coming across any mention of Greek-declension adjectives in Latin...

[Lavrentivs]

Sorry, the adjective seems to be Arpinus.

Att. 5.1: ... ut veni in Arpinas ...

So the quæstion is, why fem. and not neu?

later: quae fueramus ... in Tusculano locuti.

[Sceptra Tenens]

That quote actually makes it clear that it is neuter here, having the same form in the accusative as in the nominative.

Edit:
Make no mistake - the word is arpinas, -atis. My cursory search shows that it can be masculine (when referring to a person) or neuter (when referring to a residence).

[Damoetas]

There are some other nouns like that in Latin, like the name Maecēnās, Maecēnātis, m. (which comes from Etruscan).

[adrianus]

Et haec adjectiva latinè habes in -as -atis terminata unâ cum Arpinas -atis:
Along with Arpinas -atis, you also have these adjectives ending in -as -atis in Latin:

Assisinas -atis seu Asisinas -atis "of Assisi (the town of Assisi/Assise)" from Assisium or Asisium in Latin.
demarchisas -antis "having served as a Greek demarch" (-antis, not -atis, though)
primas -atis "of the highest rank"
quoias -atis (id est, quojas -atis) "of what place?"

and almost 800 latin nouns ending -as -atis.
et octingenti proxima nomina sic terminantur.