"good faith, fair dealing, freedom from intent to deceive," by 1838, English pluralization of bona fide, as though the Latin phrase were a noun. Sense of "guarantees of good faith" is by 1944. The opposite is mala fides "bad faith, intent to deceive."
mid-15c., of a day, "clear, fair, calm," from Old French serein and directly from Latin serenus "peaceful, calm, clear, unclouded" (of weather); figuratively "cheerful, glad, tranquil"(from PIE root *ksero- "dry," source also of Greek xeros "dry, arid;" see xerasia).
In English, the word has been applied to persons, characters, etc. since 1630s: "tranquil, unruffled." Related: Serenely. Middle English also had serenous (mid-15c.), of places, "having clear, fair weather."
French, "God and my right," the watchword of Richard I at the Battle of Gisors (1195), adopted as the motto on the royal arms of England. The "right" was Edward's claim to the crown of France upon the death of his uncle, Charles the Fair, king of France, without male issue.