Entry from OED Online

Also cod's wallop. [Origin unknown.] 

    Nonsense, drivel.
 
  1963 Radio Times 17 Oct. 52/2 Just branding a programme as ‘rubbish’, ‘tripe’, orthere are a lot of these‘codswallop’, gives little indication of what moved the viewer to write. 1966 J. PORTER Sour Cream vi. 72 It had all seemed a load of old cod's wallop to me at the time, but those lecturers were experts. 1966 A. PRIOR Operators vi. 64 All that stuff about mutual respect between police and criminal was a load of old codswallop. 1970 Peace News 30 Jan. 6/1, I hope you will stop publishing cod's wallop of the type which appeared..in Roger Moody's article.