In everyday British speech codswallop is an emphatic way of saying that something is nonsense:
The aim of Carrefour Backstage was to persuade us all that the chain was a thoroughly French enterprise. Shopping there is a patriotic act, was the message. This is codswallop.
The Times, 12 Oct. 2015.
It’s common and has been so at least since the 1950s. Its first known appearances were in a BBC radio comedy programme, Hancock’s Half Hour, written by Ray Galton and Alan Simpson. An episode from December 1959, The Poetry Society, had Tony saying “What a load of rubbish. ... I have never heard such unadulterated codswallop in all my life.” The scriptwriters had a keen ear for demotic speech and we may reasonably assume it was already well known in the spoken language.
For a popular and everyday word it’s astonishingly difficult to give any clear idea where it comes from. It’s almost as though it has dropped from the heavens fully formed.
If you look around, you will find an often-repeated tale connecting it with a nineteenth-century manufacturer of soft drinks, Hiram Codd. Despite his archetypal American first name, he was British. In the 1870s, he designed and patented a method of sealing a glass bottle by means of a ball in its neck. The Codd bottle was an immediate success, though it is long obsolete in Britain and surviving examples are highly collectable. One unexpected problem was that children smashed the bottles to use the glass balls as marbles.
Those who tell the tale suggest that drinkers who preferred their tipple to have alcohol in it were dismissive of Mr Codd’s soft drinks. As beer was often called wallop, they referred sneeringly to the fizzy drink as Codd’s wallop, and the phrase later spread its meaning to anything considered to be rubbish.
This explanation is now dismissed out of hand by the experts, not least because of the long gap between the heyday of the bottle and the appearance of the word codswallop. Few people were ever aware that the bottle had been invented by a man named Codd. If it were the origin, why isn’t it spelled coddswallop?
Some have argued it may be from cods, an old term for the testicles that’s from the Anglo-Saxon sense of cod, a bag (hence scrotum). Or, as Eric Partridge has suggested, the testicular association may be with cobblers, a rude expression of incredulity, which is from rhyming slang: cobbler’s awls = balls. The second part might then be wallop in the dialect sense of chattering or scolding.
One other possibility is worth a mention. Several references have been made online of cow’s wallop or cow’s dollop, said to be a country term for bovine excrement, thus making codswallop a close relative of bullshit. Wallop is claimed to be the noise the cowpat makes when it hits the ground, but that’s surely too strong a word; however, dollop is relevant enough and one reader has told me that he knew cow’s dollop from childhood in Surrey. I can’t find an historical reference, though I did turn up cow walloper, an old Somerset disparaging term for a peasant or agricultural worker. It’s conceivable, though unlikely, that cow’s dollop became cod’s dollop and hence codswallop.