PLACID
placid, quiet, still, tranquil, smooth, unruffled
(adjective) (of a body of water) free from disturbance by heavy waves; “a ribbon of sand between the angry sea and the placid bay”; “the quiet waters of a lagoon”; “a lake of tranquil blue water reflecting a tranquil blue sky”; “a smooth channel crossing”; “scarcely a ripple on the still water”; “unruffled water”
equable, even-tempered, good-tempered, placid
(adjective) not easily irritated; “an equable temper”; “not everyone shared his placid temperament”; “remained placid despite the repeated delays”
Source: WordNet® 3.1
Etymology
Adjective
placid (comparative placider, )
calm and quiet; peaceful; tranquil
Source: Wiktionary
Plac"id, a. Etym: [L. placidus, originally, pleasing, mild, from
placere to please: cf. F. placide. See Please.]
Definition: Pleased; contented; unruffied; undisturbed; serene; peaceful;
tranquil; quiet; gentle. "That placid aspect and meek regard."
Milton. "Sleeping . . . the placid sleep of infancy." Macaulay.
Source: Webster’s Unabridged Dictionary 1913 Edition