www.alphadictionary.com

Function: verb
Etymology: Middle English walopen to gallop, from Old French (Picard dialect) waloper
Date: 1579
intransitive verb
1 : to boil noisily 2 a : to move with reckless or disorganized haste : advance in a headlong rush b : wallow, flounder
transitive verb
1 a : to thrash soundly : lambaste b : to beat by a wide margin : trounce 2 : to hit with force : sock
— wal·lop·er noun

I associate that word with "pack(s) a wallop."

Wallop is a noun as used by Gail. That is the most familiar way I know it. For such a strange, almost slang, word it surely has a lot of definitions. See Wordnik.