xerasia n.

mid-13c., from Medieval Latin elixir "philosopher's stone," believed by alchemists to transmute baser metals into gold and/or to cure diseases and prolong life, from Arabic al-iksir "the philosopher's stone," probably from late Greek xerion "powder for drying wounds," from xeros "dry" (see xerasia). Later in medical use for "a tincture with more than one base." General sense of "strong tonic" is 1590s; used for quack medicines from at least 1630s.

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."