A daring, resolute fellow. In Germany and Flanders the boldest dogs used to hunt the boar, having a kind of buff doublet buttoned on their bodies. -- "Definition taken from The 1811 Dictionary of the Vulgar Tongue, originally by Francis Grose." Found this at fromoldbooks.org, which included a note that Rubens had painted dogs in such hunting gear. I would like to find one to show you all. (Wait till I get back to my own computer, though. You know how wacky holiday surfing is on strange rigs.)
Also found there: "dog's soup" = rain water; "to blush like a blue dog" = not blushing at all; "dog's portion" = a lick and a smell, said of one who admires women from a distance.