porkpie vs porkpie hat

porkpie and porkpie hat both are nouns.

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

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

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

porkpie (noun)	porkpie hat (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