What does this English joke mean?

The lawyer asked the coroner how to prove that the man was dead at that time.

But when he asked some questions about the basis of death, the coroner said he didn't read it.

The lawyer said that you didn't examine the body carefully, but you still issued a death certificate, didn't you?

The coroner said, let's think of it this way. Although the bottle containing the man's brain is on my desk, I think it is still possible for him to go out and become a lawyer.

It means that the coroner called the lawyer a disguised brainless.