"demagogue, one who arouses the emotions of a disorderly crowd," 1842, agent noun from rabble-rousing, which is attested by 1802 as an adjective (in Sydney Smith), by 1933 as a noun; see rabble (n.1) + rouse (v.).
c. 1300, "pack of animals" (a sense now obsolete), of uncertain origin, but possibly related to Middle English rablen "to gabble, speak in a rapid, confused manner" (mid-14c.), which is probably imitative of hurry, noise, and confusion (compare Middle Dutch rabbelen, Low German rabbeln "to chatter").
The meaning "tumultuous crowd of vulgar, noisy people" is from late 14c., probably a back-formation from the Middle English verb. It was applied contemptuously to the common or low part of any populace, regardless of tumult, by 1550s.
mid-15c., rousen, intransitive, probably from Anglo-French or Old French reuser, ruser; Middle English Compendium compares 16c. French rousee "abrupt movement." Sometimes also said to be from Latin recusare "refuse, decline," with loss of the medial -c-. Originally in English a technical term in hawking, "to shaking the feathers of the body," but like many medieval hawking and hunting terms it is of obscure origin.
The sense of "cause game to rise from cover or lair" is from 1520s. The word became general from 16c. in the figurative, transitive, meaning "stir up, cause to start up by noise or clamor, provoke to activity; waken from torpor or inaction" (1580s); that of "to awaken, cause to start from slumber or repose" is recorded by 1590s. Related: Roused; rousing.
Others are reading
<a href="https://www.etymonline.com/word/rabble-rouser">Etymology of rabble-rouser by etymonline</a>
Harper, D. (n.d.). Etymology of rabble-rouser. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/rabble-rouser