Green’s Dictionary of Slang

1. (US) rubbish, nonsense.

1901		W. Irwin Love Sonnets of a Hoodlum II n.p.: O goo-goo eye, how glassy glazed thou art To freeze my spinach solid when we meet, And keep thy Willie on the anxious seat Like a bum Dago on an apple cart!
1930		J.P. McEvoy Hollywood Girl 205: It’s a flop and then I says to him, in other words I say it’s spinach.
1934		A. Woollcott While Rome Burns 304: This [...] reticence [...] will [...] be described by certain temperaments as [...] good taste [...] I say it’s spinach.
1965		J.P. Carstairs Concrete Kimono 150: Don’t give me those big innocent eyes. Strictly for the birds—the old spinacho! In other words, Alfalfa!

2. a term of contempt.

1940

E. O’Neill Iceman Cometh Act I: No lip out of you, neither, you Dutch spinach!