postman's knock

postman's knock (countable and uncountable, plural postman's knocks)

1. (countable) A double knock at the door to indicate the delivery of letters or telegrams.
   • 1897, Bram Stoker, Dracula, Westminster [London]: Archibald Constable and Company, […], →OCLC:

     Whilst he was speaking we were startled by a knock at the hall door, the double postman's knock of the telegraph boy.

2. (uncountable) Synonym of post office (“a party game involving the exchange of kisses”).