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