What was lack of skill?

The doctor was expected to show ordinary skill. But there were two different features.

Lack of ordinary care: First, like any person, the doctor had to show reasonable care when going about his work.

Lack of professional care: Secondly, the doctor had to follow the standards of his profession. Since judges (or in England juries) could not easily assess this, the law in all countries would only find the doctor liable to pay damages or to be punished by the criminal law if he committed a very clear or serious breach of professional standards