mourn v.

Proto-Indo-European root meaning "to remember." 

It forms all or part of: commemorate; commemoration; mourn; memo; memoir; memorable; memorandum; memorial; memorious; memorize; memory; remember.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit smarati "remembers;" Avestan mimara "mindful;" Greek merimna "care, thought," mermeros "causing anxiety, mischievous, baneful;" Latin memoria "memory, remembrance, faculty of remembering," memor "mindful, remembering;" Serbo-Croatian mariti "to care for;" Welsh marth "sadness, anxiety;" Old Norse Mimir, name of the giant who guards the Well of Wisdom; Old English gemimor "known," murnan "to mourn, remember sorrowfully;" Dutch mijmeren "to ponder."

"feeling or expression of sorrow, sadness, or grief," c. 1200, from Old English murnung "complaint, grief, act of lamenting," verbal noun from mourn (v.). Meaning "customary dress or garment worn by mourners" is from 1650s (mourning habit is from late 14c.). The North American mourning dove (1820) is so called for its call.