radical, n. — Green’s Dictionary of Slang

radical n.

[so called from its being the favourite breakfast of the radical Henry ‘Orator’ Hunt (1773–1835)]

roasted corn.

1835		G. Kent Modern Flash Dict. 27: Radical – Hunt’s breakfast powder, roasted corn.
1848		Flash Dict. in Sinks of London Laid Open [as cit. 1835].