A demagogue (/ˈdɛməˌɡɒɡ/; from Ancient Greek δημαγωγός (dēmagōgós) 'popular leader, mob leader'; from Ancient Greek δῆμος (dêmos) 'people, populace' and ἀγωγός (agōgós) 'leading, guiding'), or rabble-rouser, is a political leader in a democracy who gains popularity by arousing the common people against elites.  Over time, the term acquired a negative connotation, now describing a political leader who manipulates popular sentiment for their own gain, often through emotional rhetoric and appeals to prejudice. 

According to the Oxford English Dictionary, the word "demagogue" first appeared in English in the mid-17th century, specifically around 1649.  While the word has Greek roots, it likely entered English via French, where "demagoge" was in use earlier.  The OED notes that the meaning of the word was already evolving in English at this time, with some early citations using it in a neutral sense (as a popular leader) and others in a negative sense (as a leader who manipulates the public).  The verb form, "demagogue", is recorded slightly later by the OED, with the earliest evidence dating to 1656 in the writings of political theorist James Harrington. 

Examples of "demagogue" used in a sentence:  
Again, his adversaries used to call him a dangerous demagogue. 
Again, his adversaries used to call him a dangerous demagogue. 
He was an enthusiastic, but a fickle and ambitious demagogue, and he achieved a better reputation as a writer.
He was deservedly respected in the House of Commons; seldom has an agitator been so little of a demagogue. 
Maley is playing the demagogue, which is a dirty trick. 

Just the facts:

DEMAGOGUE Definition & Meaning - Merriam-Webster

DEMAGOGUE Synonyms: 46 Similar and Opposite Words | Merriam-Webster Thesaurus