unteachableadjective
/ˌʌnˈtiː.tʃ.ə.bəl//ˌʌnˈtiː.tʃ.ə.bəl/A person or a subject or skill that is unteachable cannot be taught:
The dyslexic author had once been considered "unteachable" at school.
Good salesmen have the unteachable quality of enthusiasm.