placid adj.

c. 1300, plesen, "to please or satisfy (a deity), propitiate, appease," from Old French plaisir "to please, give pleasure to, satisfy" (11c., Modern French plaire, the form of which is perhaps due to analogy of faire), from Latin placere "to be acceptable, be liked, be approved," related to placare "to soothe, quiet" (source of Spanish placer, Italian piacere), from PIE *pl(e)hk- "to agree, be pleasant," with cognates in Tocharian plak- "to agree," plaki "permission."

By mid-14c. as "satisfy (a person), be agreeable to, be satisfactory or acceptable; to be satisfied." Meaning "to delight, attract (someone), amuse, entertain, excite agreeable sensations in" in English is from late 14c. Inverted use for "to be pleased, be satisfied" parallels the evolution of like (v.).

Impersonal constructions with it, followed by an object and originally dative are common from mid-14c. Intransitive sense of "to like, choose, think fit" (do as you please) is recorded from c. 1500; imperative use (please do this), is recorded from 1620s (as please to), was probably a shortening of if it please (you) (late 14c.).

This impersonal construction with the indirect object of the person has given way in more familiar use to a personal construction, the original dative you, in if you please, for example, being now taken as the subject. The word in this sense was formerly common in polite request, may it please you, or if it please you, or, elliptically, please you : a mode of speech still common in addressing a judge or persons of rank or position : as, may it please the court ; if it please your honor ; please your worship ; etc. [Century Dictionary] 

Verbs for "please" supply the stereotype polite word ("Please come in," short for may it please you to ...) in many languages (French, Italian), "But more widespread is the use of the first singular of a verb for 'ask, request' " [Buck, who cites German bitte, Polish proszę, etc.]. Spanish favor is short for hace el favor "do the favor." Danish has in this sense vær saa god, literally "be so good."

"tranquility, peacefulness, quietness," 1610s, from Latin placiditatem (nominative placiditas), from placidus "peaceful, quiet, gentle, calm" (see placid).