"liquid food," 1650s, from French soupe "soup, broth" (13c.), from Late Latin suppa "bread soaked in broth," from a Germanic source (cf. Middle Dutch sop "sop, broth"), from Proto-Germanic *sup-, from PIE *sub-, from root *seue- (2) "to take liquid" (see sup (v.2)).
Primordial soup is from a concept first expressed 1929 by J.B.S. Haldane. Soup to nuts "everything" is from 1910. Soup-kitchen, "public establishment supported by voluntary contributions, for preparing and serving soup to the poor at no cost" is attested from 1839. In Ireland, souper meant "Protestant clergyman seeking to make proselytes by dispensing soup in charity" (1854).