flog a dead horse

/F L AA1 G AH0 D EH0 D AO0 R S/
verb
  1. 1

    To attempt to get extra work out of a ship's crew during the dead horse period.

  2. 2

    To attempt to get more out of something that cannot give more; to attempt to arouse fresh interest in something that is either hopeless or already settled.

Source: https://en.wiktionary.org/wiki/flog%20a%20dead%20horse