Automotive and aviation usage is attested at least since 1925 (in Popular Mechanics) — possibly from 1921 — with even early citations linking it to supercharged. Therefore, it is often contended that soup is a clipping of super.
However, this is antedated by usage in horse racing cant that applies the term to horses, and by a US Navy euphemism for drunkenness. The former is attested in Webster New International Dictionary (1909) and the latter in Our Navy (May 1915). Both may be figurative interpretations of soup as a liquid food item, although other origins cannot be discounted.
- (horse racing, cant) of horses injected with something to make them run faster or change their temperament (19th-early 20th century)
- (US Navy, slang) drunk
- (of an engine, a motor vehicle or another device) modified for higher performance (likely derived from the horse-racing term)
- (Rhode Island, Massachusetts, Connecticut, slang) excited