so-and-so vs such-and-such

so-and-so

noun

  • A placeholder name for a person or thing, used when a name is not known; a generic name. 

  • Some thing or things. 

  • Used in place of epithets such as SOB, bastard, etc.. 

such-and-such

noun

  • A placeholder or generic thing. 

det

  • indefinite; some or other, not stated 

How often have the words so-and-so and such-and-such occurred in a corpus of books? (source: Google Ngram Viewer )