To exert formidable power. In modern English to wallop means to thrash, and in noun form, a heavy blow, but originally the verb meant to boil with a noisy, bubbling sound, and the noun also was slang for ale. Glyndebourne, site of a summer opera festival in England, perpetuates the last meaning in the name of its restaurant, Nether Wallop (Lower Ale). The verb pack in this expression means “to deliver.” The term dates from the early twentieth century. Eugene O’Neill used it literally in his play The Hairy Ape (1922): “He packa da wallop, I tella you.” Figuratively it appears in such locutions as, “The candidate’s speech really packed a punch.”