overkill vs elbow grease

overkill and elbow grease both are nouns.

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

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

overkill: any effort that seems to go farther than would be necessary to achieve its goal
elbow grease: use of physical or mental energy; hard work

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

overkill (noun)	elbow grease (noun)
any effort that seems to go farther than would be necessary to achieve its goal	use of physical or mental energy; hard work
the capability to obliterate a target with more weapons (especially nuclear weapons) than are required