Unsupportable
Part of speech: adjective
Definition: Not able to be supported or endured.
Unbearable
Part of speech: adjective
Definition: so unpleasant or painful as to be unendurable
Example sentence: I know April, May and June are a few unbearable months, and working out in a gym and sweating in such dirty hot, sticky, humid weather puts me off. The best way is to swim. I feel so fresh and rejuvenated after swimming, and I believe it's one of the best mode to fitness during summers.