Unpayable is an adjective that describes something that cannot be paid, often due to excessive or unreasonable expenses. However, there are several synonyms which can be used interchangeably with unpayable such as impracticable, unfeasible or impossible. Interestingly, unpayable can also be replaced with words like insurmountable, intractable, or insoluble, which all imply a sense of intractability or un-resolvability. Another similar word to unpayable is infeasible, which implies something that is impossible to achieve due to logistical or logical reasons. Overall, there are several synonyms for unpayable, and they can all be used to describe a range of situations where payment is not feasible or impossible.