This is a portmanteau word meaning a dollop of swill; it refers to maybe-edible substances in institutional food such as entrees served in school cafeterias or military messes. The word is all the more delicious for being said out loud in company, since it contains in its sound the implication 'swallow'. If you're young and hungry, you'll eat it, despite appearances.
Swallop may be subcategorized: processed pork, eg spam, is swine swallop, fish sticks are cod swallop, meat loaf is moo swallop. Reconstituted potatoes or eggs, warm congealed cereal, stew, mystery meat, and other such offerings merely are swallop.