From Longman Dictionary of Contemporary Englishwallopwal‧lop /ˈwɒləp $ ˈwɑː-/ verb [transitive] informalHIT to hit someone or something very hard, especially with your hand —wallop noun [singular]→ See Verb table
Examples from the Corpus
wallop• So one day, I got a newspaper, I rolled it up and I walloped her like mad.• He walloped his head on a beam.• And do you honestly think that I normally allow females to wallop me across the face?• Then Caroline would have to wallop my back to stimulate a return to normal breathing.• Either leave or wallop them; one or the other.Origin wallop (1300-1400) Old North French waloper, from Old French galoper; → GALLOP1 