thorn vs pain in the ass

thorn and pain in the ass both are nouns.

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

As nouns, pain in the ass is a hypernym of thorn; that is, pain in the ass is a word with a broader meaning than thorn:

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

Other hypernyms of thorn include annoyance, bother, botheration, infliction, nuisance, pain, pain in the neck.

thorn (noun)	pain in the ass (noun)
something that causes irritation and annoyance	something or someone that causes trouble; a source of unhappiness
a Germanic character of runic origin
a small sharp-pointed tip resembling a spike on a stem or leaf