Mightn't is a contraction of "might not" which means the possibility of something not happening or being the case. There are several synonyms for mightn't which include "could not," "may not," "would not," and "should not." All these words imply that an event or action is not likely to happen or is impossible. These synonyms can be used interchangeably depending on the context in which they are being used. They all share the same sense of negation and express uncertainty about the possibility of a particular outcome. Using synonyms for mightn't can add variety to your writing and make your language more vibrant.