welkin (n.)

"sky" (poetic), Old English wolcen "cloud," also "sky, heavens," from Proto-Germanic *wulk- (source also of Old Saxon wolkan, Old Frisian wolken, Middle Dutch wolke, Dutch wolk, Old High German wolka, German Wolke "cloud"), perhaps from PIE *welg- "wet" (source also of Lithuanian vilgyti "to moisten," Old Church Slavonic vlaga "moisture," Czech vlhky "damp"); but Boutkan rejects this and finds no good IE etymology.