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1.
Something that doesn't make sense and sounds like utter nonsense can be described as codswallop, also, in some places of devon, there are cults that use the walloping of a cod as a method of induction into their sect...
That's just utter codswallop you pillock or Is thou ready for the codswallop young initiate?!?
by ChibiGarrick September 30, 2002
 
2.
Used to denote the fallaciousness of any statement/idea.

Orig. Old English, where the 'cods' referred to the cock and balls, thus codswallop was jiz, spunk, cum, semen.
Person 1. "War is a necessary evil."
Person 2. "Codswallop! You fucking idiot, have you ever actually read a book?"
by edjog November 04, 2005
 
3.
Often said by Hagrid in Harry Potter movies and book. The actual definition is believed to be a sack tap done by a pissed off fish after it is caught.
John: I love watching America's Funniest Home Videos, especially when someone gets Codswalloped by the pissed off fish they caught
by Suikodenfan88 July 31, 2010