Are there any indeclinable adjectives?

Question

I had until recently believed that only nouns could be "declinable" versus "indeclinable": most nouns follow set declensions patterns, while a few (mostly foreign, like Abraham from Hebrew, but some native, like fas) look the same in every form.

However, brianpck corrected me in his comment on this answer, pointing out that the adjective nēquam is also indeclinable.

Are there any other indeclinable adjectives out there?

Answer 1

There certainly are other indeclinable adjectives:

• damnas
• frugi (gramatically, this is really a "dative of service")
• nēquam
• potis (though pote sometimes occurs in the neuter)
• quot (also: aliquot)
• tŏt (also: totidem)
• And, after tres, almost every numerical adjective, e.g. quattuor, decem, viginti, mille, etc.

Allen & Greenough discusses this, along with many of the above examples, in §122, along with a mention of defective and common adjectives.