nth vs such-and-such a …

nth

adj
  • Occurring at a relatively large but unspecified position in a series. 

  • Occurring at position n in a sequence. 

noun
  • The item at position n in a sequence. 

  • The item in a relatively large but unspecified position in a series. 

such-and-such a …

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