old salt vs Jack

old salt and Jack both are nouns.

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

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

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

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