You use if the worst comes to the worst to say what you might do if a situation develops in the most unfavourable way possible. The form if worst comes to worst is also used, mainly in American English. If the worst comes to the worst I guess I can always ring Jean He was asked whether he would walk out if the worst came to the worst
You use at worst or at the worst to indicate that you are mentioning the worst thing that might happen in a situation. At best Nella would be an invalid; at worst she would die
the weakest effort or poorest achievement one is capable of; "it was the worst he had ever done on a test"
Worst is used to form the superlative of compound adjectives beginning with `bad' and `badly'. For example, the superlative of `badly-affected' is `worst-affected'