privy vs under one’s hat

privy

adj
  • With knowledge of; party to; let in on. 

  • Private, exclusive; not public; one's own. 

noun
  • An outdoor facility for urination and defecation, whether open (latrine) or enclosed (outhouse). 

  • A partaker; one having an interest in an action, contract, etc. to which he is not himself a party. 

  • A lavatory: a room with a toilet. 

  • A toilet: a fixture used for urination and defecation. 

under one’s hat

How often have the words privy and under one’s hat occurred in a corpus of books? (source: Google Ngram Viewer )