Green’s Dictionary of Slang

1. (US) to defecate.

1977		C. McFadden Serial 14: The last time she’d gone there, a gull had dumped on her shrimp Louie.
1982		G.V. Higgins Patriot Game (1985) 103: No pigeon in his right mind would dump on Seats Lobianco.
2001		N. Barlay Hooky Gear 197: He one of them relics with a face more dump on than a buildin-site wheelbarrow.

2. in fig. use, to treat badly, i.e. shit on under shit v.

1992

R.G. Barrett White Shoes 42: They [...] didn’t like to see a good bloke being dumped on.