What is the meaning of [Off with his head]

Literal meaning. That is, ‘chop off his head’. It is now usually used humorously as a means of mildly reproaching someone.

Shakespeare used the phrase many times in his plays and I can find no record of any earlier usage; for example, in Henry VI Part III, 1592:

Off with his head, and set it on York gates;
So York may overlook the town of York.