The phrase 'soup up' is slang that basically means 'to increase the power of' something. Why do we use 'soup' here? It seems similar to the phrases 'beef up' and 'pep up', which literally come from the foods beef and pepper, so is it as simple as the fact that it's a variation on those phrases that uses 'soup' as the food? But why soup? Is soup considered particularly 'hearty'?
The OED postulates that to soup up may have been influenced by the super- prefix, such as is seen in words like supercharged:
Notable citations both early and late include: