Definitions of worst:

• noun:   the weakest effort or poorest achievement one is capable of

  Example: "It was the worst he had ever done on a test"

• noun:   the greatest damage or wickedness of which one is capable

  Example: "The invaders did their worst"

• noun:   the least favorable outcome

  Example: "The worst that could happen"

• verb:   defeat thoroughly
• adjective:   (superlative of `bad') most wanting in quality or value or condition

  Example: "The worst player on the team"

• adverb:   to the highest degree of inferiority or badness

  Example: "She suffered worst of all"

• name:  A surname (very rare: popularity rank in the U.S.: #76005)