Why is "dog" in "underdog"?

Dog seems a strange word to choose for this concept. Does anyone know anything more than Dictionary.com can tell me?

Origin: 1875–80, Americanism; under- + dog

Etymonline has a similar take, but it also makes reference to top dog, another incomprehensibly endowed noun with an even less gratifying origin entry:

Origin: 1885–1900

Where can I find the etymology of these words?

Solution 1:

I found this from googling, so its validity may be questionable, but it came up fairly often.

It seems that both underdog and top dog originated from dog fighting which went on in the 19th century. The losing dog ended up on the bottom, or under the winner, who was on top.

My reference is The Times of India.

Solution 2:

Underdog

The "under dog" was the dog who lost a fight (as opposed to the winning "top dog"), and underdog has become idiomatic for the inferior person or party, or the one fighting a larger adversary.

The OED says it originates in the US and their earliest citation is the British Daily Telegraph of 1887:

There is an indefinable expression in his face and figure of having been vanquished, of having succumbed, of having been ‘under-dog’ as the saying is.

I found an earlier citation from The Popular Science Monthly of April 1882, in an article called "Has Science Yet Found a New Basis for Morality?" by Professor Goldwin Smith and quoting Dr. Van Buren Denslow:

But, if the under dog in the social fight runs away with a bone in violation of superior force, the top dog runs after him bellowing, "Thou shalt not steal," and all the other top dogs unite in bellowing, "This is divine law, and not dog law"; the verdict of the top dog, so far as law, religion, and other forms of brute force are concerned, settles the question.

And on the next page, Smith summarises:

It appears that between one state and the other there may be an interval in which the question will be not between the moral and the immoral, but between the top and the under dog.

Top dog

The "top dog" was similarly the dog that won the fight, and now figuratively refers to a winner or the best person, party or thing.

The earliest OED citation is from a 1900 edition of The Speaker: A Review of Politics:

The most popular argument in favour of the war is that it will make the individual Briton top dog in South Africa.

Again, the above Popular Science Monthly antedates this, as does another I found in The Spy of the Rebellion (1883) by Allan Pinkerton:

"Fo' de Lawd, gemmen, I'se hopin an' prayin' de No'thun folks will be de top dog in dis wrastle, an' ef eber dis niggah hes a chance to gib yu'uns a a helpin' han', yu' kin bet a hoss agin' a coon-skin he'll do it; but I hope an' trus my missus not be boddered."

Variation: upper dog

The OED's first citation for the rare variation upper dog is from 1903 by G. Bowles in Hansard's Parliamentary Debates:

If it came to a question of force, we should always be the ‘upper dog’ in Persia.

I found an earlier example in the April 1881 Popular Science Monthly in "Some Notes on a Doctor's Liability" by Oliver E. Lyman:

Quackery was again the upper dog. But the struggle was not over. Although on top, the other dog had a vicious grip upon him. The empiric was still unable to recover compensation from delinquent patients.