Coup de grâce can be translated from French as stroke of grace or blow of mercy. A coup de grâce is a death blow to end the suffering of a severely wounded person or animal. It may be a mercy killing of civilians or soldiers, friends or enemies, with or without the sufferer's consent.
"Yu gonplei Ste Odon" means "Your fight is over" in Trigedasleng. Trigedasleng, or Trig, is a constructed language spoken by the earth-born people, the clans of Mid-Atlantic United States, known as Grounders. Trigedasleng translates to "forest language", it originated with the Trikru clan and doesn't have its own writing system. It was often used earlier but mentioning here as the subtitles shown this spelling for the first time here, and the subtitle wrongly changed i to L "Yu gonple'L' Ste Odon"