plac·id 
(plăsĭd)
(plăsĭd)adj.
1.
a. Undisturbed by tumult or disorder; calm or quiet: led a placid life.
b. Not agitated physically; not disturbed: a placid stream.
2. Not easily upset or excited; mild or complacent: a placid demeanor. See Synonyms at calm.
[Latin placidus, from placēre, to please; see plāk-1 in the Appendix of Indo-European roots.]
pla·cidi·ty (plə-sĭdĭ-tē), placid·ness (plăsĭd-nĭs) n.
placid·ly adv.