The Collaborative International Dictionary of English v.0.48:

Dak \Dak\ (d[add]k or d[aum]k), n. [Hind. [dsdot][=a]k.]
   Post; mail; also, the mail or postal arrangements; -- spelt
   also dawk, and dauk. [India]
   [1913 Webster]

   Dak boat, a mail boat. --Percy Smith.

   Dak bungalow, a traveler's rest-house at the end of a dak
      stage.

   To travel by dak, to travel by relays of palanquins or
      other carriage, as fast as the post along a road.