Definition
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.
How People Actually Use It
UK usage 1) To try to carry on when there really is no hope of continuation. To not know when to give up. 2) A unique sexual practise that combines Necrophillia, beastiality and sadomasacism into one fun activity.
"Leeds struggled to avoid relegation but they were flogging a dead horse."
Source: Urban Dictionary