An unbearable situation or experience is one that's extremely difficult or painful to deal with.
The word "unbearable" is often used to describe something that feels too much to handle. Imagine a really bad headache - it's not just annoying, but actually stops you from doing things because it hurts so much. That's an example of unbearable pain. Or maybe you're in a crowded place on a hot summer day and the noise and heat feel overwhelming - that can be unbearable too.
So unpleasant or painful as to be unendurable
