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.
Alteration of -d- to -th- begins late 15c., though such pronunciation may be older (see father (n.)). In nautical use, as an adjective, "toward the wind" (opposed to lee).
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.