What’s the origin of the word pooch as a synonym for dog? m-w.com says “origin unknown,” but what do they know? I defer to the Master.
Of course I first defer to my fellow Dopers… 
It’s more refined than “Pooper”?
OED says “etymology is obscure”; I’d hazard a guess that it’s onomatopoeic for the way people talk to their dogs.
“Pooch” is also an archaic variant of “pouch,” but I don’t think that gets us anywhere…
Does OED say if its origins are English? I can’t think of what language it could have come from.
The OED doesn’t take a stand on the etymology.
Barry Popik has found earlier cites for the slang term(1912) than the OED, always in US papers.
Some members of the American Dialect Society, on the mailing list, have speculated that it comes from Scots dialect “pouch/pooch” but no firm evidence.