The literal meaning of the word demagogue is "leader of the people".

It gained negative connotations because in Ancient Greece, (where the word originated), 'the people' were considered to be an uneducated mob whose demands were not to be taken seriously - thus anyone who spoke, or made demands, on their behalf were considered as rabble rousers by the ruling classes.

Thus the idea that a demagogue appeals to the emotions, fears, prejudices and ignorance of the lower classes was how the ruling class described anyone who espoused the cause of the common man as they recognised the danger that demagogues posed for the establishment.