A wallop is a hard punch or another strong hit. A boxer sometimes wins a match with one single, well-aimed wallop to their opponent.
A wallop is not just a physical blow, but it can also be a huge effect or impact. A principal's strongly worded lecture to a group of bullies might pack a wallop, for example, resulting in a major change in behavior. Use this word as a verb and it means "punch" or "defeat completely," so you'll frequently hear it in sports reporting: "The Wizards score the last points of the game and wallop the Pacers."