early 14c., pesful, peesfull, paisful, "inclined to peace, friendly, pacific;" mid-14c. as "tranquil, calm, full of peace;" from peace + -ful. In reference to nonviolent methods of effecting social change, it is attested from 1876. Related: Peacefully; peacefulness. Peaceful coexistence (1920) originally was in regard to Soviet policy toward the capitalist West.
mid-15c., serenite, "fair, calm, clear weather," from Old French (Modern French sérénité) and directly from Latin serenitatem (nominative serenitas) "clearness, serenity," from serenus (see serene). Of persons, "tranquility of mind or temper," by 1590s. Also formerly used as a title of honor for kings (mid-15c.), probably from the similar use of Latin serenitas, which was applied to Roman emperors and later to popes.
c. 1200, "excused, exempt, free, clear" (of debt, obligation, penalty, etc.), from Old French quite, quitte "free, clear, entire, at liberty; discharged; unmarried," and directly from Medieval Latin quitus, quittus, from Latin quietus "free" (in Medieval Latin "free from war, debts, etc."), also "calm, resting" (from PIE root *kweie- "to rest, be quiet").
From mid-13c. as "deprived of." From c. 1300 of real property, "exempt from taxes or other dues or claims."
"release or discharge from debt, a final clearing of accounts," 1530s, short for Medieval Latin phrase quietus est "he is quit," from quietus "free" (in Medieval Latin "free from war, debts, etc."), also "calm, resting" (from PIE root *kweie- "to rest, be quiet"). The full Latin phrase was used in English from early 15c. Hence, "death" (i.e. "final discharge"), c. 1600. Latin quies also was used for "the peace of death."
in medicine, "tending to calm or soothe," early 15c. (Chauliac), sedatif, from Old French sedatif and directly from Medieval Latin sedativus "calming, allaying," from sedat-, past participle stem of Latin sedare, causative of sedere "to sit" (from PIE root *sed- (1) "to sit").
The noun derivative meaning "a sedative drug" is attested by 1797, short for sedative salt, etc.; earlier it was used in a figurative or non-medical sense (1785), "whatever soothes or allays."