Meaning of “worst” - Learner’s Dictionary

worst

noun uk us /wɜːst/
the worst

B1 the most unpleasant or difficult thing, person, or situation:

I've made some mistakes in the past, but this is definitely the worst.
at worst

used to say what the most unpleasant or difficult situation could possibly be:

At worst, we might lose our money.
if the worst comes to the worst UK (US if worse/worst comes to worst)

if a situation develops in the most unpleasant or difficult way

(Definition of “worst noun” from the Cambridge Learner’s Dictionary © Cambridge University Press)