Why is the ninth month called September?
c.1600, 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- "to shake, tremble" (cf. Sanskrit trprah "hasty," Old Church Slavonic trepetati "to tremble"), related to *trem- (see tremble).
trepidation trep·i·da·tion (trěp'ĭ-dā'shən)
An involuntary trembling or quivering.
A state of anxious fear; apprehension.