Unthinkable describes something so extreme, shocking, or impossible that your mind struggles to even imagine it happening. When something is unthinkable, it goes against everything you expect or believe could occur.

Before airplanes were invented, the idea of flying across the ocean in a few hours would have been unthinkable. In a well-run classroom, it's unthinkable that students would just get up and leave whenever they felt like it. For most people, deliberately hurting a friend is unthinkable because it contradicts their basic values about friendship.

The word often appears when someone wants to emphasize how wrong or unlikely something is: “It's unthinkable that we'd cancel the school play after months of rehearsal.” Sometimes people use it to mean they refuse to consider an option: “Giving up is unthinkable.”

What makes something unthinkable is that the idea itself feels wrong or impossible to seriously consider. The unthinkable sits at the edge of what seems real or acceptable. Yet history shows that many unthinkable things do eventually happen, which is why people sometimes say “the unthinkable has occurred” when faced with a shocking event.