1704, "woman who affects or upholds modesty in conduct and thought in a degree considered rigid and excessive," from French prude "excessively prim or demure woman," first recorded in Molière.
Perhaps it is a false back-formation or an ellipsis of preudefemme "a discreet, modest woman," from Old French prodefame "noblewoman, gentlewoman; wife, consort," the fem. equivalent of prudhomme "a brave man" (see proud (adj.)). Or perhaps the French noun is from the French adjective prude "prudish," from Old French prude, prode, preude, which however is attested only in a laudatory sense, "good, virtuous, modest," a feminine form of the adjective preux. Also occasionally as an adjective in English 18c.; the application of the noun to a man was still considered rare at the end of 19c.
adjectival word-forming element, Old English -isc "of the nativity or country of," in later use "of the nature or character of," from Proto-Germanic suffix *-iska- (cognates: Old Saxon -isk, Old Frisian -sk, Old Norse -iskr, Swedish and Danish -sk, Dutch -sch, Old High German -isc, German -isch, Gothic -isks), cognate with Greek diminutive suffix -iskos. In its oldest forms with altered stem vowel (French, Welsh). The Germanic suffix was borrowed into Italian and Spanish (-esco) and French (-esque). Colloquially attached to hours to denote approximation, 1916.