Green’s Dictionary of Slang

1. an unpopular person.

1945		in T. Shibutani Derelicts of Company K (1978) 270: Someone started referring to him as ‘Major Cowshit’ […] These expressions enjoyed wide usage, and from this point on ‘Major Cowshit’ came to symbolize all the pettiness and anything else obnoxious that took place.
1962		L. McMurtry Leaving Cheyenne 69: Here come two cowshits.
2006		bobstruth.blogspot 19 Apr. [blog] Durham County’s District Attorney, Mike Nifong, is the biggest piece of cowshit that has hit the fan recently.

2. nonsense, rubbish.

1967		C.P. Taylor Happy Days Are Here Again (1968) 174: Cowshit!
2020		G. Armstrong Young Team 10: Yi huv tae take aw these stories wae a pinch ae fuckin salt. Most [...] ir fuckin total cow dung.