I don't believe anyone's 100% sure of the original meaning. But during WW2 the expression certainly took on a whole new meaning. England, particularly London, was being blitzed by the German's almost every night, most builders were away fighting, so emergency repairs were carried out by men and sometimes even women who were not qualified, so the work was generally under-par. These workers became known as Jerry or Gerry builders. As you know, Gerry/Jerry was British slang for German. There, a little history lesson for the 'kids' here. lol
PS Sorry, no more off-topic. I promise.
Just to correct your little history lesson, Jerry. The term "jerry built" has been in use since the 19th century.
"The phrases 'jerry built'/'jerry building'/'jerry builder' have been around since at least 1869, when 'jerry built' was defined in the Lonsdale Glossary:
"Jerry-built, slightly, or unsubstantially built."
By 1901, the term began to be used figuratively - a sure sign of acceptance into the general language; for example, The Daily Chronicle, in August that year printed this opinion:
"In an age of jerry-built books it is refreshing to come across a volume that has taken forty years to compile."
The derivation is unknown.
What we do know is that the term has nothing to do with the UK slang term for German - Jerry/Gerry. This is of WWI origin and the citations above pre-date that. As always when a phrase's origin is unknown people like to guess, so here goes. It is possible that the term derives from the slang term jerrycummumble or jerrymumble. This was defined in the 1811 version of Francis Grose's Classical Dictionary of the Vulgar Tongue:
"JERRYCUMMUMBLE. To shake, towzle, or tumble about."
(just for info. Over and out)