diffident (adj.)

c. 1400, "distrust, want of confidence, doubt of the ability or disposition of others," from Latin diffidentia "mistrust, distrust, want of confidence," from diffidere "to mistrust, lack confidence," from dis- "away" (see dis-) + fidere "to trust" (from PIE root *bheidh- "to trust, confide, persuade"). The opposite of confidence. Original sense (distrust of others) is obsolete; the modern sense is of "distrust of oneself, want of confidence in one's ability, worth, or fitness" (1650s), hence "retiring disposition, modest reserve."  

Diffidence is a defect: it is an undue distrust of self, with fear of being censured for failure, tending to unfit one for duty. [Century Dictionary]

"characterized by bold opposition or antagonism," 1837, from French défiant, present participle of défier "to challenge, defy, provoke" (see defy). Related: Defiantly. Compare diffident.