The Return of the Jungle

The origin of the term AB is unclear. The most widely quoted story has it that of Hiram A, 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 A Bottle.

B is a slang term for beer, and beer drinkers would certainly be disdainful of bottled soft drinks. This slang term dates from around the early to mid 20th century. Eric Partridge, in A Dictionary of Slang, claims it as serviceman's slang and dates it from the 1930s. An early example of it in print comes in J[ohn] B[oynton] Priestley's Three men in new suits, 1945:

"It's drink... Booze or B... Nine times out of ten... you wake up in the morning... with the usual hangover."

It's not difficult to see how a soft drink in a A Bottle could have come to be called AB.