n.
Function: noun
Date: 14th century
1 a : a sharp blow : RAP , HIT <a loud knock on the door> b (1) : a severe misfortune or hardship (2) : SETBACK , REVERSAL
2 a : a pounding noise b : a sharp repetitive metallic noise caused by abnormal ignition in an automobile engine
3 : a harsh and often petty criticism <the knock on him was that he couldn't handle the pressure>