Words related to weld
"to spring, rise, gush," Old English wiellan (Anglian wællan), causative of weallan "to boil, bubble up, rise (in reference to a river)" (class VII strong verb; past tense weoll, past participle weallen), from Proto-Germanic *wellanan "to roll" (source also of Old Saxon wallan, Old Norse vella, Old Frisian walla, Old High German wallan, German wallen, Gothic wulan "to bubble, boil"), from PIE root *wel- (3) "to turn, revolve," on notion of "roiling or bubbling water."
Or possibly the adjective is from or influenced by Middle English noun gaudegrene (early 14c.), name of a yellowish-green color or pigment, originally of dye obtained from the weld plant (see weld (n.1)). This Germanic plant-name became gaude in Old French, and thus the Middle English word. Under this theory, the sense shifted from "weld-dye" to "bright ornamentation."
As a noun, "feast, festival" 1650s, from gaudy day "day of rejoicing" (1560s).