incognito vs such-and-such

incognito

noun
  • One unknown or in disguise, or under an assumed character or name. 

  • The assumption of disguise or of a feigned character; the state of being in disguise or not recognized. 

adv
  • Without revealing one's identity. 

adj
  • Without being known; in an assumed character, or under an assumed title; in disguise. 

such-and-such

noun
  • A placeholder or generic thing. 

det
  • indefinite; some or other, not stated 

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