Old English weder, from Proto-Germanic *wedran (cf. Old Saxon wedar, Old Norse veðr, Old Frisian, Middle Dutch, Dutch weder, Old High German wetar, German Wetter "storm, wind, weather"), from PIE *we-dhro-, "weather," from root *we- "to blow" (see wind (n.)). Spelling with -th- first appeared 15c., though pronunciation may be much older.
Weather-beaten is from 1520s. Under the weather "indisposed" is from 1827. 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.