old man vs wormwood

old man and wormwood both are nouns.

Word	Noun	Adjective	Verb	Adverb
old man	Yes	No	No	No
wormwood	Yes	No	No	No

As nouns, wormwood is a hypernym of old man; that is, wormwood is a word with a broader meaning than old man:

old man: aromatic herb of temperate Eurasia and North Africa having a bitter taste used in making the liqueur absinthe
wormwood: any of several low composite herbs of the genera Artemisia or Seriphidium

old man (noun)	wormwood (noun)
an informal term for your father	any of several low composite herbs of the genera Artemisia or Seriphidium
a man who is very old
a familiar term of address for a man
(slang) boss
aromatic herb of temperate Eurasia and North Africa having a bitter taste used in making the liqueur absinthe