There are several synonyms for the word "uncancellable" that can be used in different contexts. The word "irrevocable" can be used to describe something that cannot be undone or cancelled. "Non-refundable" is another word that can be used to describe a payment or agreement that cannot be cancelled and the money cannot be returned. "Inalterable" is another synonym that can be used to describe something that cannot be changed or cancelled. "Immutable" is another word that can be used to describe a situation or agreement that is unable to be modified or cancelled. Essentially, all of these synonyms can be used interchangeably with "uncancellable" depending on the context and specific meaning intended.