Green’s Dictionary of Slang

a excl. stating that one is ineligible for a given duty or command or that one forbids it, e.g. fen play!, no playing.

1853		Dickens Bleak House (1991) 224: I’m fly, [...] But fen larks, you know. Stow hooking it!
1864		Bartlett Dict. Americanisms (1877) 212: A prohibitory exclamation used by boys in their games; as, ‘Fen play’, i.e. I forbid you to play.
1885		‘’Arry on the Elections’ in Punch 12 Dec. 277/2: Fen brickbats, though, Charlie, old bloater.