bad luck vs hard knocks

bad luck and hard knocks both are nouns.

Word	Noun	Adjective	Verb	Adverb
bad luck	Yes	No	No	No
hard knocks	Yes	No	No	No

As nouns, hard knocks is a hyponym of bad luck; that is, hard knocks is a word with a more specific, narrower meaning than bad luck:

Other hyponyms of bad luck include weakness, adversity, hardship, gutter, sewer, toilet, hard cheese.

bad luck (noun)	hard knocks (noun)
unnecessary and unforeseen trouble resulting from an unfortunate event	a state of misfortune or affliction
an unpredictable outcome that is unfortunate
an unfortunate state resulting from unfavorable outcomes