porkpie hat vs porkpie

porkpie hat and porkpie both are nouns.

Word	Noun	Adjective	Verb	Adverb
porkpie hat	Yes	No	No	No
porkpie	Yes	No	No	No

As nouns, porkpie hat and porkpie are synonyms defined as:

porkpie hat and porkpie: man's hat with a low, flat crown and a snap brim

porkpie hat (noun)	porkpie (noun)
man's hat with a low, flat crown and a snap brim	man's hat with a low, flat crown and a snap brim