It is an adjective because it is a word that is used to describe a noun.
Adjective
giggety Atrocious is an adjective. The adverb is atrociously and the noun is atrocity.
You have atrocious hair today! ((Your hair looks terrible)) I can't believe that atrocious color was included in the painting ((that color in the painting is terrible.)) ect ect.
Atrocious means it is something unbelievable or silly.
Atrocious and terrible are synonymous, yes.
The duration of Atrocious - film - is 1.25 hours.
The way the press treat most artists is atrocious.
Atrocious - film - was created on 2010-10-15.
The part of speech for this particular word is a noun.
part of speech
The part of speech for "answer" is a noun.
what part of speech is beneath