so-and-so vs dirty dog

so-and-so and dirty dog both are nouns.

Word	Noun	Adjective	Verb	Adverb
so-and-so	Yes	No	No	No
dirty dog	Yes	No	No	No

As nouns, so-and-so and dirty dog are synonyms defined as:

so-and-so and dirty dog: a person who is deemed to be despicable or contemptible

Other synonyms of so-and-so include bum, crumb, git, lowlife, puke, rat, rotter, scum bag, scumbag, skunk, stinker, stinkpot.

so-and-so (noun)	dirty dog (noun)
a person who is deemed to be despicable or contemptible	a person who is deemed to be despicable or contemptible