Green’s Dictionary of Slang

kingfish n.

[the original Kingfish was the populist Governor and Senator Huey P. Long (1893–1935) of Louisiana; Long, who declared that he ‘looked around at the little fishes present and said “I’m the Kingfish”’, fought his campaigns on the slogan ‘Everyman a King but no man wears a crown’. The name was also given to a character in the hit US radio show Amos ’n’ Andy]

1. (US) a political leader or ‘boss’.

[US]J.J. Finerty Criminalese 33: Kingfish — an organizer or backer.
H.P. Long Every Man a King 277: In the course of our several political battles we [...] termed various of our political enemies the ‘Kingfish’.
[US]M. Braun Judas Tree (1983) 129: Yeager accused him of being the mastermind behind the robberies and the political kingfish of Virginia City.
[US]S. King It (1987) 910: Dis here’s de Kingfish! I is de Kingfish in Derry.

2. in political sense, an important person.

[UK]in K. Richards Life 352: Oh boy, we nailed a kingfish and his sidekick now, buddy!