The Collaborative International Dictionary of English v.0.48:
Deathblow \Death"blow`\, n. A mortal or crushing blow; a stroke or event which kills or destroys. [1913 Webster] The deathblow of my hope. --Byron. [1913 Webster]WordNet (r) 3.0 (2006):
deathblow n 1: the blow that kills (usually mercifully) [syn: deathblow, coup de grace]