1640s, agent noun from agitate (v.); originally "elected representative of the common soldiers in Cromwell's army," who brought grievances (chiefly over lack of pay) to their officers and Parliament.
Political sense is first recorded 1734, and negative overtones began with its association with Irish patriots such as Daniel O'Connell (1775-1847). Historically, in American English, often with outside and referring to people who stir up a supposedly contented class or race. Latin agitator meant "a driver, a charioteer."
