'underdog' originates from English, formed as a compound of the element 'under' (Old English 'under') and 'dog' (Old English 'docga'/'dog'), where 'under' meant 'below' or 'beneath' and 'dog' meant 'dog'.

'underdog' developed in colloquial and nautical/slang contexts in the late 18th to early 19th century with a literal sense of a dog that was beneath another; by the 19th century it was used figuratively for a person at a disadvantage and later became common in sporting and general usage.

Initially it referred literally to a dog in a lower position (for example, in a fight); over time it shifted to the figurative meaning 'a person or team expected to lose' and then broadened to any disadvantaged or less powerful party.