Green’s Dictionary of Slang

1. (US) a French-born immigrant.

1989

H. Rawson Dict. of Invective (1991) 229: pea soup, a Frenchman.

2. (US, also Johnny Peasoup, pea, souper) a French-Canadian; thus talk peasoup v., to talk in French-Canadian patois.

1896		G. Parker Pomp of Lavilettes (1913) 233: Yes, an’ dey call us Johnny Pea-soups.
1912		B. Heeney Pickanock 22: Pea-soup! I never drink with the likes of you, Pauquett!
1930		‘Dean Stiff’ Milk and Honey Route 38: A Canadian Frenchman is a ‘Canuck’ or sometimes a ‘pea soup’.
1949		Monteleone Criminal Sl. (rev. edn) 173: pea A French Canadian. [Ibid.] 218: souper A French Canadian.
1965–70		in DARE.
1978		Maledicta II:1+2 (Summer/Winter) 167: Pea-souper Any French-Canadian or Quebecker, after the Quebec dietary staple.
1983		Maledicta VII 21: Immigrant Quebeckers were a more visible group, and their staple peasoup by the 1890s resulted in johnny peasoup and the popular variants peasoup, pea souper, and French peasoup.

3. see peasouper n. (2)