1530s, of weather, 1590s, of persons, from French sérénité, from Latin serenitatem (nominative serenitas) "clearness, serenity," from serenus (see serene). Earliest use (mid-15c.) was as a title of honor for kings, probably from the similar use of Latin serenitas, applied to Roman emperors, later popes.
"gentle, quiet, undisturbed, serene, calm," 1620s, from French placide (15c.) and directly from Latin placidus "pleasing, peaceful, quiet, gentle, still, calm," from placere "to please" (see please). Related: Placidly; placidness.
Smooth-bore in reference to guns is from 1812. smooth talk (v.) is recorded from 1950. A 1599 dictionary has smoothboots "a flatterer, a faire spoken man, a cunning tongued fellow." The usual Old English form was smeðe, and there is a dialectal smeeth found in places names, such as Smithfield, Smedley.
Old English weder "air, sky; breeze, storm, tempest," from Proto-Germanic *wedra- "wind, weather" (source also of Old Saxon wedar, Old Norse veðr, Old Frisian, Middle Dutch, Dutch weder, Old High German wetar, German Wetter "storm, wind, weather"), traditionally said to be from PIE *we-dhro-, "weather" (source also of Lithuanian vėtra "storm," Old Church Slavonic vedro "good weather"), suffixed form of root *we- "to blow." But Boutkan finds this "problematic from a formal point of view" and finds only the Slavic word a likely cognate.
Greek had words for "good weather" (aithria, eudia) and words for "storm" and "winter," but no generic word for "weather" until kairos (literally "time") began to be used as such in Byzantine times. Latin tempestas "weather" (see tempest) also originally meant "time;" and words for "time" also came to mean weather in Irish (aimsir), Serbo-Croatian (vrijeme), Polish (czas), etc. Weather-report is from 1863. Weather-breeder "fine, serene day which precedes and seems to prepare a storm" is from 1650s.
Surnames Fairweather, Merriweather probably reflect disposition; medieval lists and rolls also include Foulweder, Wetweder, Strangweder.