Green’s Dictionary of Slang

1. (US) excrement; esp. as cow-flop n.; thus as a dimissive retort.

1937		D. Fuchs Low Company 73: ‘Slow as cow flop on an ice-cold day,’ muttered the jerker.
1948		in G. Legman Limerick (1953) 139: A nasty young joker named Bruce / Used to greet all his friends with a goose, / Till it came to a stop / In a handful of flop / From some bowels that were terribly loose.
1955		J.P. Donleavy Ginger Man (1958) 13: Up to our ankles in mud and cow flop.
1962		H. Livings Nil Carborundum (1963) Act III: It’s as soft as cowflop this side, Jack.
1975		T. Berger Sneaky People (1980) 184: Recalling Ralph’s walking through the cowflop.
1982		(con. 1970) J.M. Del Vecchio 13th Valley (1983) 391: My toes get colder en yesterdays cow flop.
1993		J. Mowry Six Out Seven (1994) 33: Couldn’t be no worse’n cowflops an scrap iron.
2005		E.H. Peterson Christ Plays in Ten Thousand Places 134: We used the dried cowflop for bases in our ball games.
2012		T. Pluck ‘Summer of Blind Joe Death’ in Life During Wartime (2018) 160: She left her flops on the path [...] then stood chewing her cud and waited for you to step in one.

2. in fig. use, nonsense.

1987

S. Haden Elgin Earthsong (2002) 75: For our elitism, and our selfishness, and our shameful wealth, and our hoarded secrets—all of which is the most utter cowflop.