gaudyyoudaoicibaDictYouDict[gaudy 词源字典]
gaudy: [16] Middle English had a colour term gaudy-green ‘yellowish-green’, which originally denoted ‘green produced by dye obtained from the plant dyer’s rocket, Reseda luteola’, a plant formerly known as weld [14]. The word weld came from a Germanic source which, borrowed into Old French, produced gaude – whence English gaudy-green. It has been claimed that this gaudy soon lost its literal meaning ‘produced from weld-dye’, and came to be interpreted as ‘bright’.

Other etymologists, however, favour the explanation that gaudy comes from gaud ‘joke, plaything’ [14], which was adapted from Old French gaudir ‘rejoice’, a descendant of Latin gaudēre ‘delight in’ (from which English gets joy).
[gaudy etymology, gaudy origin, 英语词源]
gaudy (adj.)youdaoicibaDictYouDict
"showy, tastelessly rich," c. 1600; earlier "joyfully festive" (1580s), probably a re-adjectivizing of gaudy (n.) "large, ornamental bead in a rosary" (early 14c.) via the noun gaud + -y (2.). In early Modern English it also could mean "full of trickery" (1520s).

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).