fifth wheel vs deadwood

fifth wheel and deadwood both are nouns.

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

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

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

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