A lot vs. Alot: 9 Grammatical Pitfalls
past participle adjective from disturb. Meaning "emotionally or mentally unstable" is from 1904.
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.).