onus vs dead weight

onus and dead weight both are nouns.

Word	Noun	Adjective	Verb	Adverb
onus	Yes	No	No	No
dead weight	Yes	No	No	No

As nouns, dead weight is a hyponym of onus; that is, dead weight is a word with a more specific, narrower meaning than onus:

Other hyponyms of onus include fardel, imposition, pill.

onus (noun)	dead weight (noun)
an onerous or difficult concern	a heavy motionless weight
	an oppressive encumbrance