past master vs expert

past master and expert both are nouns.

Word	Noun	Adjective	Verb	Adverb
past master	Yes	No	No	No
expert	Yes	Yes	No	No

As nouns, expert is a hypernym of past master; that is, expert is a word with a broader meaning than past master:

past master: someone who has long and thorough experience in a given activity
expert: a person with special knowledge or ability who performs skillfully

past master (noun)	expert (noun)
someone who has long and thorough experience in a given activity	a person with special knowledge or ability who performs skillfully
someone who was formerly a master

past master (adjective)	expert (adjective)
	having or showing knowledge and skill and aptitude
	of or relating to or requiring special knowledge to be understood