unthinkable

adj.

1. 不能考虑的, 不能接受的, 不可想像的, 难以置信的
   • It would be unthinkable to ask him to do that.
     叫他做那件事是毫无可能的。
   • It is quite unthinkable that a child like him can speak a foreign language so fluently.
     像他这样的孩子居然能流利地说一种外语, 真是令人难以置信。