bitters vs John Barleycorn

bitters and John Barleycorn both are nouns.

Word	Noun	Adjective	Verb	Adverb
bitters	Yes	No	No	No
John Barleycorn	Yes	No	No	No

As nouns, John Barleycorn is a hypernym of bitters; that is, John Barleycorn is a word with a broader meaning than bitters:

bitters: alcoholic liquor flavored with bitter herbs and roots
John Barleycorn: an alcoholic beverage that is distilled rather than fermented

Other hypernyms of bitters include booze, hard drink, hard liquor, liquor, spirits, strong drink.

bitters (noun)	John Barleycorn (noun)
alcoholic liquor flavored with bitter herbs and roots	an alcoholic beverage that is distilled rather than fermented