dead horse
Noun
Meaning
(dated, nautical) The period of work on board ship for which the seamen have been paid in advance (usually a month's wages), the end of this term being celebrated by parading a straw horse about the decks.
(Australia, rhyming slang) Tomato sauce.
Source: en.wiktionary.orgSynonyms
Examples
Alimony is like buying hay for a dead horse.
I'd call him a sadistic, hippophilic necrophile, but that would be beating a dead horse.
You cannot kickstart a dead horse.
You've got to stop whipping a dead horse sometime.
No use in flogging a dead horse.
Try to save the dead horse as if it is still alive.