prude (n.)

1704, "woman who affects or upholds modesty in a degree considered excessive," from French prude "excessively prim or demure woman," first recorded in Molière. Perhaps a false back-formation or an ellipsis of preudefemme "a discreet, modest woman," from Old French prodefame "noblewoman, gentlewoman; wife, consort," fem. equivalent of prudhomme "a brave man" (see proud); or perhaps a direct noun use of the French adjective prude "prudish," from Old French prude, prode, preude "good, virtuous, modest," a feminine form of the adjective preux. Also occasionally as an adjective in English 18c.