pain in the ass vs pain in the neck

pain in the ass and pain in the neck both are nouns.

Word	Noun	Adjective	Verb	Adverb
pain in the ass	Yes	No	No	No
pain in the neck	Yes	No	No	No

As nouns, pain in the ass and pain in the neck are synonyms defined as:

pain in the ass and pain in the neck: something or someone that causes trouble; a source of unhappiness

Other synonyms of pain in the ass include annoyance, bother, botheration, infliction, nuisance, pain.

pain in the ass (noun)	pain in the neck (noun)
something or someone that causes trouble; a source of unhappiness	something or someone that causes trouble; a source of unhappiness