In supportable means something that cannot be supported. The antonyms for the word "in supportable" would be supportable, bearable, tolerable, sustainable, tenable, credible, defensible, and justifiable. If something is supportable, it means it can be supported or sustained. If something is bearable, it means it can be tolerated or endured. Tenable and credible refer to ideas or arguments that are logical and reasonable. Defensible and justifiable refer to actions or decisions that can be explained or justified. All these antonyms are the opposite of "in supportable" which implies something that is impossible to be held or continued.