deadwood vs fifth wheel

deadwood and fifth wheel both are nouns.

Word	Noun	Adjective	Verb	Adverb
deadwood	Yes	No	No	No
fifth wheel	Yes	No	No	No

As nouns, deadwood and fifth wheel are synonyms defined as:

deadwood and fifth wheel: someone or something that is unwanted and unneeded

deadwood (noun)	fifth wheel (noun)
someone or something that is unwanted and unneeded	a steering bearing that enables the front axle of a horse-drawn wagon to rotate
a branch or a part of a tree that is dead	an extra car wheel and tire for a four-wheel vehicle
	someone or something that is unwanted and unneeded