Green’s Dictionary of Slang

in pl., nonsense, rubbish.

[1812		W. Combe Doctor Syntax, Picturesque (1868) 39: Doctor Syntax’s Song [...] With harum scarum, horum scorum – / Stew’d prunes for ever! / Stew’d prunes for ever!].
1926		Wood & Goddard Dict. Amer. Sl. 44: rot. Twaddle, nonsense, stewed prunes.