The origin of the term codswallop is unclear. The most widely quoted story has it that of Hiram Codd, an English soft drinks maker during the 1870s, who developed a technique for bottling lemonade. This process involved the insertion of a glass marble as a stopper into the neck of the bottle. When the bottle was shaken the resulting pressure from the fizzy pop forced the marble against the neck to form a seal. The device was called, not unreasonably, the Codd Bottle. Wallop is a slang term for beer, and beer drinkers would certainly be disdainful of bottled soft drinks. It's not difficult to see how a soft drink in a Codd Bottle could have come to be called codswallop.
There's no actual evidence for that derivation though. In fact neat plausibility without evidence is often the mark of the linguistic urban mythology known as folk etymology.
The most likely explanation is that it is a made up word that just sounds right for its meaning.
- from http://www.phrases.org.uk/meanings/235250.html
HP fans will tell you that Hagrid often uses this word. I am not a HP fan, and yet I know this useless fact. I know many such useless facts. More to come.