tranquil

Borrowed from Middle French tranquille, from Latin tranquillus, which joined trāns- ("across") to the root of quiēs ("rest, quiet, peace"), itself from the ancient root *kʷyeh₁-, "to rest."

The facts don't describe a shift in meaning — the word seems to have carried the sense of calm rest from its Latin form through French and into English.

It shares its root with quiēs, the Latin word for rest or peace, meaning it's kin to any English word built on that same ancient sense of quiet.