Merriam-Webster Collegiate Dictionary - knock

 I. verb  Etymology: Middle English knoken, from Old English cnocian; akin to Middle High German knochen to press  Date: before 12th century  intransitive verb  1. to strike something with a sharp blow  2. to collide with something  3.  a. bustle heard them ~ing around in the kitchen  b. wander ~ed about Europe all summer  4.  a. to make a pounding noise  b. to have engine ~  5. to find fault  transitive verb  1.  a.  (1) to strike sharply  (2) to drive, force, or make by or as if by so striking was ~ed out of the campaign  b. to set forcibly in motion with a blow  2. to cause to collide  3. to find fault with always ~ing those in authority  II. noun  Date: 14th century  1.  a. a sharp blow ; rap, hit a loud ~ 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 ~ on him was that he couldn't handle the pressure