flog a dead horse

English

flog a dead horse (third-person singular simple present flogs a dead horse, present participle flogging a dead horse, simple past and past participle flogged a dead horse)

(British, idiomatic) 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.
(dated, nautical) To attempt to get extra work out of a ship's crew during the dead horse period.

to attempt to get more out of something that cannot give more

Estonian: surnud hobust peksta, tühja tööd tegema, viljatut ideed uuesti üles võtma
Finnish: piiskata kuollutta hevosta
German: auf etwas herumreiten; seine Zeit verschwenden, sich vergeblich bemühen
Hungarian: vesztegeti az idejét, mint halottnak a csók, falra hányt borsó (literally “pea[s] thrown onto [a] wall”), feleslegesen igyekszik/erőlteti, lezárt témán lovagol
Italian: sprecare tempo, è fatica sprecata, è una battaglia persa (literally “it's a lost battle”)
Kikuyu: kũhũra maaĩ na ndĩrĩ (literally “to pound water with a mortar”)
Malay: mencari jejak dalam air (literally “to look for a trail/track in the water”)
Persian: آب در هاون کوبیدن (fa) (âb dar hâvan kubidan, literally “to grind water in a mortar”)
Polish: no equivalent term in Polish, but see robić coś na próżno impf, zrobić coś na próżno pf
Portuguese: chutar cachorro morto (pt) (literally “to kick a dead dog”), chover no molhado (pt)
Russian: решето́м во́ду носи́ть (rešetóm vódu nosítʹ, literally “carry water in a sieve”), напрасно стараться (naprasno staratʹsja, literally “try hard in vain”)
Scottish Gaelic: a' marbhadh coin mhairbh (literally “killing a dead dog”)
Spanish: machacar en hierro frío (literally “to strike cold iron”)
Sundanese: mapatahan ngojay ka meri (literally “to teach a duck how to swim”)
Turkish: havanda su dövmek (tr), boşa kürek çekmek