hot air vs restraint

hot air

noun
  • Air that has been heated, especially so as to function as the lifting agent of a hot-air balloon. 

  • Empty, confused, or exaggerated talk lacking meaning or substance; bluster. 

restraint

noun
  • something that restrains, ties, fastens or secures 

  • control or caution; reserve 

How often have the words hot air and restraint occurred in a corpus of books? (source: Google Ngram Viewer )