nadir vs hard knocks

nadir and hard knocks both are nouns.

Word	Noun	Adjective	Verb	Adverb
nadir	Yes	No	No	No
hard knocks	Yes	No	No	No

As nouns, hard knocks is a hypernym of nadir; that is, hard knocks is a word with a broader meaning than nadir:

Other hypernyms of nadir include adversity, hardship.

nadir (noun)	hard knocks (noun)
the point below the observer that is directly opposite the zenith on the imaginary sphere against which celestial bodies appear to be projected	a state of misfortune or affliction
an extreme state of adversity; the lowest point of anything