A requiem is a (usually Catholic) mass to honor the souls of the deceased. The irreligious, however, might better know the term from the somber and often eerie music composed for these occasions. Requiem was borrowed into the English language around the turn of the fourteenth century directly from church Latin, where it was a conjugation of requies, meaning "rest", as in the soul of the dead person is at rest. This is a combination of the prefix re-, meaning "again", and the root quies, or "rest" (the etymon of quiet, through Old English quiete, also "rest"). So the re- is a bit redundant; there are a lot of useless affixes in Latin. Re- is from Proto-Italic wre, with the same definition. That could be from a similar Proto-Indo-European word, but there's no widely accepted reconstruction. Quies, meanwhile, can be traced to the PIE root kwyeh, which still meant "rest" but also "peace". Usage of requiem in literature has been decreasing since a maximum in the middle of the nineteenth century.
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