Why can「うんざり」be used directly preceding「だ」/「です」?


I recently came across 「うんざり」(which dictionaries describe as an "adverb" and
"する-verb"), and cannot figure out why it works in sentences where it directly
precedes 「だ」/「です」.

An example is: 「彼女の態度にはうんざりだ。」

I can understand the sentence, but can't figure out how that usage fits into
the definition of either "adverb" or "する-verb". Isn't it being used
adjectivally?

How does 「うんざり」fit either or those roles in such sentences?

