while, whilom, tranquility, requiem

According to the Indo-European Roots Index of The American Heritage® Dictionary of the English Language, these four words ultimately come from the Indo-European root *kwei-, or also *kwye-, which means “to rest” or “to be quiet”. The root became *hwilo- in Germanic, and it later produced the Old English words *hwil and *hwilum, where the words while and whilom respectively come from. The change in the root, formerly a /k/ sound and later a voiceless fricative /h/ is explained by Grimm’s Law.

On the other hand, the same Indo-European root survived into the Latin language, where it shaped the word tranquillus, meaning something like “accros the rest”. In Middle English, it transformed again into tranquill, and ultimately into Modern English tranquillity. Again in Latin and by means of the same Indo-European root, the word quies appeared with the same meaning of rest and quietness. And this is where the word requiem (an hymn to those that are already resting forever) comes from.