airmail vs special delivery

airmail

noun
  • The items of mail so carried. 

  • The system of conveying mail using aircraft. 

verb
  • To send mail by air. 

  • To (unintentionally) throw the ball well over a fielder's head where that fielder is unable to make a play on the ball. 

special delivery

noun
  • A kind of postal service in which, for an extra fee, letters and packages are delivered in a highly expedited manner by a special courier. 

  • A particular posted letter or package which is delivered in this manner; a particular act of conveying such letters or packages to one or more recipients. 

  • Something—whether desirable or undesirable—which is intentionally given to a specific individual or which an individual receives rapidly and unexpectedly; the personalized, direct quality of the transmission of such an item. 

How often have the words airmail and special delivery occurred in a corpus of books? (source: Google Ngram Viewer )