the term 'underdog' originated in the mid-19th century from the sport of dogfighting. it literally referred to the dog that was overpowered and pinned beneath its winning opponent. this vivid, literal image of a defeated animal was then semantically broadened. it first extended to any contestant in a fight or competition considered least likely to win. finally, the term evolved further into its modern sociological sense, describing any person or group in a position of disadvantage within a social or economic hierarchy, thereby completing its journey from a concrete sporting term to an abstract metaphor for societal status.

Structure: under + dog under (from old english under, meaning "beneath, lower in position or rank"); dog (from old english docga, meaning "a powerful breed of canine").