You searched for: “unthinkable
inconceivable, unthinkable
inconceivable (in" kuhn SEE vuh buhl) (adjective)
Impossible to understand: It was inconceivable that the professor wrote such an unsubstantiated article about the president of the university.
unthinkable (un THING kuh buhl) (adjective)
1. Impossible to be understood by the mind or contrary to reasonable expectations: It was unthinkable that the children should be expected to walk so far.
2. Difficult to imagine or to believe: It was unthinkable that the teacher was leaving the school before the end of the semester.

Tamika found the mathematical formula on the examination inconceivable; however, it was unthinkable that she would fail the exam because of this one problem.