c.1600, from Middle French trepidation (15c.) and directly from Latin trepidationem (nominative trepidatio) "agitation, alarm, trembling," noun of action from past participle stem of trepidare "to tremble, hurry," from trepidus "alarmed, scared," from PIE *trep- (1) "to shake, tremble" (cognates: Sanskrit trprah "hasty," Old Church Slavonic trepetati "to tremble"), related to *trem- (see tremble, v.).