Noah Webster
Placid
PLAC'ID, adjective [Latin placidus, from placo, to appease.]
1. Gentle; quiet; undisturbed; equable; as a placid motion of the spirits.
2. Serene; mild; unruffled; indicating peace of mind; as a placid countenance or smile.
3. Calm; tranquil; serene; not stormy; as a placid sky.
4. Calm; quiet; unruffled; as a placid stream.