A person or a subject or skill that is unteachable cannot be taught.

The dyslexic author had once been considered "unteachable" at school. 这位有阅读障碍的作者曾经在学校被认为是“不可教”的。

Good salesmen have the unteachable quality of enthusiasm. 优秀的推销员都有一种与生俱来、无法传授予他人的热情。