what is oxymoron?
An oxymoron is a phrase combining two opposite ideas.
Note: The word "oxymoron" itself is an oxymoron. The Greek word "oxy" means "sharp" and "moron" means "dull".
Traditionally a shrimp is a small delicacy. The word "jumbo" refers to something very large. Therefore combining the words "jumbo" and "shrimp" creates an oxymoron which is two contradictory terms together. The idea of a small creature being large is contradictory. Similarly a vacation is a holiday and if somebody takes a "working vacation" then they are not truly on holiday. Also, the food in a cafeteria has been prepared earlier, not immediately which suggests it is not fresh. By their nature, oxymorons cause the reader to think a little bit more so create the effect of encouraging the reader to look for deeper meaning.