monkey jacket vs shell jacket

monkey jacket and shell jacket both are nouns.

Word	Noun	Adjective	Verb	Adverb
monkey jacket	Yes	No	No	No
shell jacket	Yes	No	No	No

As nouns, monkey jacket and shell jacket are synonyms defined as:

monkey jacket and shell jacket: waist-length jacket tapering to a point at the back; worn by officers in the mess for formal dinners

Other synonyms of monkey jacket include mess jacket.

monkey jacket (noun)	shell jacket (noun)
waist-length jacket tapering to a point at the back; worn by officers in the mess for formal dinners	waist-length jacket tapering to a point at the back; worn by officers in the mess for formal dinners