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.