Source : Webster's Revised Unabridged Dictionary (1913)

Placid \Plac"id\, a. [L. placidus, originally, pleasing, mild,
   from placere to please: cf. F. placide. See {Please}.]
   Pleased; contented; unruffied; undisturbed; serene; peaceful;
   tranquil; quiet; gentle. ``That placid aspect and meek
   regard.'' --Milton. ``Sleeping . . . the placid sleep of
   infancy.'' --Macaulay.

Source : WordNet®

placid
     adj 1: free from disturbance; "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" [syn: {quiet}, {still}, {tranquil}, {unruffled}]
     2: not easily irritated; "an equable temper"; "not everyone
        shared his placid temperament"; "remained placid despite
        the repeated delays" [syn: {equable}, {even-tempered}, {good-tempered}]
     3: without untoward incident or disruption; "a placid
        existence"; "quiet times" [syn: {quiet}]
     4: taking life easy; "an easygoing man rarely stirred to
        anger"; "an air of placid sufficiency" [syn: {easygoing}]