A punch-up is a fight, usually one where people hit each other with their fists. It is informal, and you hear it much more in British English than in American English. In the US, people would more often just say a fight. A punch-up is often short and messy — outside a pub, after a match — not a long, planned battle. People sometimes say “a bit of a punch-up,” which can sound almost light, though someone may still get hurt. The noun comes from the verb punch up, meaning to start hitting. There is another punch up that means to make a piece of writing stronger and more lively. That is a different use. This word, punch-up, is the fight.