Undefendable is an adjective that implies something is unable to be protected or justified against criticism, attack, or scrutiny. In other words, it's a vulnerable position, without any valid excuse or explanation. The synonyms of undefendable are numerous, including indefensible, untenable, insupportable, unjustifiable, inexcusable, inexplicable, unexplainable, inarguable, irrefutable, and incontrovertible. Each of these words shares similar connotations of something that is impossible to support, defend, explain or justify. The use of such words indicates the sheer magnitude of the shortcomings of an argument, logic, or action, and the need for those involved to reconsider and take corrective measures.