disturb (v.)

late 13c. distourben, "to frighten, alarm, break up the tranquility of;" 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). Related: Disturbed; disturbing; disturbingly.

Middle English also had the verb as distourblen, from Old French destorbler; hence also distourbler (n.) "one who disturbs or incites" (late 14c.).