c.1300, murdre, from Old English morðor (plural morþras) "secret killing of a person, unlawful killing," also "mortal sin, crime; punishment, torment, misery," from Proto-Germanic *murthra- (cf. Goth maurþr, and, from a variant form of the same root, Old Saxon morth, Old Frisian morth, Old Norse morð, Middle Dutch moort, Dutch moord, German Mord "murder"), from PIE *mrtro-, from root *mer- "to die" (see mortal (adj.)). The spelling with -d- probably reflects influence of Anglo-French murdre, from Old French mordre, from Medieval Latin murdrum, from the Germanic root.
Viking custom, typical of Germanic, distinguished morð (Old Norse) "secret slaughter," from vig (Old Norse) "slaying." The former involved concealment, or slaying a man by night or when asleep, and was a heinous crime. The latter was not a disgrace, if the killer acknowledged his deed, but he was subject to vengeance or demand for compensation.
Weakened sense of "very unpleasant situation" is from 1878.