secret vs under one’s hat

secret

verb
  • To make or keep secret. 

  • To hide secretly. 

adj
  • Being or kept hidden. 

noun
  • Private seclusion. 

  • A piece of knowledge that is hidden and intended to be kept hidden. 

  • The key or principle by which something is made clear; the knack. 

  • A form of steel skullcap. 

  • Something not understood or known. 

  • Any prayer spoken inaudibly and not aloud; especially, one of the prayers in the Tridentine Mass, immediately following the "orate, fratres", said inaudibly by the celebrant. 

under one’s hat

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