What is the meaning of [The Devil incarnate]

The Devil in human form.

In 16th century England the devil was considered to be a real living entity that roamed the land and was able to transform itself, or at least inhabit, other living beings. The expression ‘the devil in human form’ wasn’t a metaphor as we might use it now but an actuality. The first reference to it that I know of in print is in the collection of tragic poems The Mirour for Magistrates, 1578:

A wicked wretch, a kinseman most vnkynde [unkind],
A Deuil incarnate, all deuilishly enclynde [inclined]

The Devil incarnateThe expression must have been in common usage in the late 16th century as Shakespeare also used it, without any explanation, in King Henry V, 1598, and Titus Andronicus, 1588:

Henry V – Boy: Yes, that a’ did; and said they were devils

Titus Andronicus – LUCIUS: O worthy Goth, this is the incarnate devil
That robb’d Andronicus of his good hand.