[1] : The Collaborative International Dictionary of English v.0.48

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.
   1913 webster
see also:
please 

[2] : WordNet (r) 2.0

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
see also:
quiet still tranquil unruffled equable even-tempered 
good-tempered easygoing 

[3] : Moby Thesaurus II by Grady Ward, 1.0

75 moby thesaurus words for "placid":
   affliction, at rest, bane, besetment, bother, botheration, calm,
   cloistered, collected, composed, cool, coolheaded, curse, detached,
   disease, dwindling, easy, easygoing, ebbing, epidemic,
   even-tenored, exasperation, halcyon, hushed, impassive,
   inexcitable, infestation, invasion, irenic, irritant, isolated,
   moldering, nuisance, outbreak, pacific, peaceable, peaceful, pes,
   pest, pestilence, philosophical, plague, poised, quiescent, quiet,
   rash, ravage, reposeful, reposing, restful, resting, scourge,
   secluded, self-possessed, sequestered, sequestrated, serene,
   sheltered, smooth, still, still as death, stillish, stilly, stoic,
   stolid, subsiding, tranquil, unagitated, undisturbed, unmoved,
   unperturbed, unruffled, unstirring, untroubled, waning

Placid >> P Words
Placid, definition of term: Placid
placid_pag1.html

Lake Placid >> L Words
Lake Placid, definition of term: Lake Placid
lake+placid_pag1.html