Green’s Dictionary of Slang

(US) a public orator, a rabble-rouser.

1911		Down News & Down Times (KS) 20 Apr. 6/1: The calls from the soap-boxers [...] were fast becoming a demonstration. ‘Tell me some more; let them roar’.
1913		J. London Valley of the Moon (1914) 174: You’d make a good soap-boxer [...] if only you’d get the kinks straightened out in your reasoning.
1923		Indep. Record (Hewlena, MT) 27 June 4/1: The Mayor insists it is an infringement of the right of ‘free speech’ to keep the soapboxers out of the streets.
1932		L. Adamic Laughing in the Jungle 87: The soap-boxer’s bull voice boomed over the crowd.
1944		Dly News (NY) 11 July 55/1: ‘When you get a guy with a platform like that you won’t stop with making him king of the soapboxers. You’ll make him President of the United States’.
1956		Pantagraph (Bloomington, IL) 3 June 8/2: What happened to the dear old line of soapboxers on Hyde Parl Corner.
1964		Pasadena Indep. (CA) 10 Dec. 13/5: I’m not arguing against the soapboxer [...] I’ve fought for freedom of speech.
1976		Morn. Call (Allentown, PA) 26 Aug. n.p.: [A] black jamaican with a red flag was haranguing the crowd. From the crowd came a roaring American voice [...] attacking the communist soapboxer.