means The contestant, team, or person expected to lose — the one with the odds stacked against them, who wins our sympathy precisely because they probably shouldn't win at all.

from A 19th-century image straight from the dog pit: in a brutal dogfight, the dog forced beneath was the "underdog," while the dominant one stood on top as the "top dog." The cruelty of the original scene faded, but the picture stuck — the creature pinned down, fighting up from below — and the word leapt from the pit into politics, sport, and every contest where the little guy is supposed to fold.