Placid (adjective)

1. Calm, tranquil, and undisturbed.
2. Not easily excited or perturbed; not showing emotion.
3. Smooth and unruffled; calm.

Origin:

Late 16th century: from latin placidus "calm, peaceful", from placere "to please".

Examples:

1. The lake was placid and still, reflecting the blue sky.
2. He had a placid demeanor and never seemed to get angry.
3. The baby was sleeping placidly in its crib.
4. The patient had a placid expression on her face during the surgery.
5. It was a placid day, with not a cloud in the sky.