Green’s Dictionary of Slang

1. a Congregationalist.

1922		D.L. Cady Rhymes of Vermont (1926) 17: My! such a peaceful, fambly day, It makes you Congos Quakers [DA].
1950		WELS.
1968–70		in DARE.
1972		A. Chipper Aussie Swearers Guide 61: The first thing any full-blooded Aussie does when handling an amen snorter is shorten his denomination to size and add an ‘O’. As in Bappo, Congo, Metho, Presbo, Protto, Salvo, etc.

2. the Congregational Church.

1979

B. Gutcheon New Girls (1982) 20: The other girls go to Congo – that’s the Congregational Church.