c. 1300, defien, "to renounce one's allegiance;" mid-14c., "to challenge to fight, dare to meet in combat;" from Old French defier, desfier "to challenge, defy, provoke; renounce (a belief), repudiate (a vow, etc.)," from Vulgar Latin *disfidare "renounce one's faith" (in Medieval Latin diffidare), from Latin dis- "away" (see dis-) + fidus "faithful" (from PIE root *bheidh- "to trust, confide, persuade"). By 1670s as "dare (someone) to do something (that the challenger believes cannot or will not be done)."