disturb (v.) Look up disturb at Dictionary.com
c.1300, "to stop or hinder," from Old French destorber (Old North French distourber) and directly from Latin disturbare "throw into disorder," from dis- "completely" (see dis-) + turbare "to disorder, disturb," from turba "turmoil" (see turbid).

Meaning "to frighten" is late 13c.; that of "to stir up, agitate" is c.1300. Related: Disturbed; disturbing; disturbingly. Middle English also had distourbler (n.) "one who disturbs or incites" (late 14c.).