Green’s Dictionary of Slang

flounder n.²

[SE flounder, i.e. the fishing industry]

(US) a Newfoundlander.

1909		‘The Lang. of Crooks’ in Wash. Post 20 June 4/1: [paraphrasing J. Sullivan] A flounder is a native of Newfoundland.
1949		Monteleone Criminal Sl. (rev. edn).