hen is on, a, phr. — Green’s Dictionary of Slang

hen is on, a phr.

[ety. unknown]

(US) something important is about to happen.

1878		in Wash. Hist. Quarterly XVIII 192: Keep cool, boys, there’s a hen on [DA].
1896		Ade Artie (1963) 60: ‘I got a hen on.’ ‘What is it?’.
1908		K. McGaffey Sorrows of a Show Girl Ch. xviii: Something has gone wrong, or there is a big hen on.
1937		(con. WW1) E.C. Parsons Great Adventure 142: There was no question but that there was a big hen on, but nobody knew what it was to be.