Synonyms for Placid (same or very similar meaning)
WordNet sense 1 (not easily irritated):
equable, even-tempered, good-tempered
WordNet sense 2 (having an easygoing and cheerful disposition):
good-natured
WordNet sense 3 (living without undue worry; taking life easy):
easygoing
WordNet sense 4 (satisfied or showing satisfaction with things as they are):
content, contented, content
WordNet sense 5 ((of a body of water) free from disturbance by heavy waves):
quiet, smooth, still, tranquil, unruffled
WordNet sense 6 ((of weather) free from storm or wind):
calm
From the ODE community, based on WordNet
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