Word Origin

AHD: REQUIEM, from Latin,accusative of requies [ rest, the first word of the mass for the dead ]

BJB, the Oxford English Dictionary includes in it's entry for each word not only a definition of the word but it's history. That is why it is considered the "last word" in the description of the English language. The OED is on line for subscription fee of $500 a year (ouch!). It is cheaper to saunter down to your local library and look the word up.

Middle English, from Latin (first word of the introit of the requiem mass), accusative of requies rest, from re- + quies quiet, rest