city boy vs city slicker

city boy and city slicker both are nouns.

Word	Noun	Adjective	Verb	Adverb
city boy	Yes	No	No	No
city slicker	Yes	No	No	No

As nouns, city boy and city slicker are synonyms defined as:

city boy and city slicker: a city dweller with sophisticated manners and clothing

city boy (noun)	city slicker (noun)
a city dweller with sophisticated manners and clothing	a city dweller with sophisticated manners and clothing