From Old English 'morthor' (secret killing), from PIE *mer- (to die) — same root as Latin 'mors' and 'mortal.'
The unlawful premeditated killing of one human being by another; an act or instance of such killing.
From Old English 'morþor' (secret killing, unlawful killing), from Proto-Germanic *murþrą (murder, death), from Proto-Indo-European *mr̥-tro- (killing), a suffixed form of *mer- (to die, to disappear). The word is thus a native Germanic inheritance, not a Latin borrowing, though it shares the same PIE root as Latin 'mors' (death) and the entire 'mortal/mortuary/mortify' family. The spelling with 'u' rather than 'o' reflects Middle English dialectal variation; the earlier 'morþor' would have yielded 'morther' in standard development. Key
Chaucer's 'Nun's Priest's Tale' contains the proverb 'mordre wol out' — murder will out, meaning that murder cannot be concealed forever. This proverbial wisdom, already ancient in Chaucer's day, reflects a deep folk belief that secret killing generates a disturbance in the moral order that inevitably reveals itself. The saying survives in modern English as 'murder will out.'