The crossword solution for the term 'REALLY WALLOP' is 'DECK'. The term 'DECK' is a noun that refers to a flat surface, usually outdoors, that is used for activities such as sunbathing, entertaining, or dining. It can also refer to the floor of a ship or the area of a ship where the crew works. In this context, 'DECK' could be used to describe the act of delivering a powerful blow, as in 'decking someone'.