Jack vs old salt

Jack and old salt both are nouns.

Word	Noun	Adjective	Verb	Adverb
Jack	Yes	No	No	No
old salt	Yes	No	No	No

As nouns, Jack and old salt are synonyms defined as:

Other synonyms of Jack include gob, Jack-tar, mariner, sea dog, seafarer, seaman, tar.

Jack (noun)	old salt (noun)
a man who serves as a sailor	a man who serves as a sailor