dak vs surface mail

dak

noun
  • A post system by means of transport relays of horses stationed at intervals along a route or network, carrying mail and passengers. 

  • A dak bungalow. 

verb
  • To suddenly pull down someone's pants as a prank; to pants. 

surface mail

noun
  • Mail which is transported by motor vehicle, by railway, or by seacraft, but not by aircraft. 

How often have the words dak and surface mail occurred in a corpus of books? (source: Google Ngram Viewer )