Middle English mornen, from Old English murnan "to feel or express sorrow, grief, or regret; bemoan, long after," also "be anxious about, be careful" (class III strong verb; past tense mearn, past participle murnen), from Proto-Germanic *murnan "to remember sorrowfully" (source also of Old Saxon mornon, Old High German mornen, Gothic maurnan "to mourn," Old Norse morna "to pine away"), probably from suffixed form of PIE root *(s)mer- (1) "to remember." Or, if the Old Norse sense is the base one, from *mer- "to die, wither."
Specifically, "to lament the death of" from c. 1300. Meaning "display the conventional appearance of grieving for a period following the death of someone" is from 1520s. Related: Mourned; mourning.