diligence vs elbow grease

diligence and elbow grease both are nouns.

Word	Noun	Adjective	Verb	Adverb
diligence	Yes	No	No	No
elbow grease	Yes	No	No	No

As nouns, elbow grease is a hypernym of diligence; that is, elbow grease is a word with a broader meaning than diligence:

Other hypernyms of diligence include effort, exertion, sweat, travail.

diligence (noun)	elbow grease (noun)
a diligent effort	use of physical or mental energy; hard work
persevering determination to perform a task
conscientiousness in paying proper attention to a task; giving the degree of care required in a given situation