I too like to explorer the origins of words we use but never think about why or what they actually mean or how they came about.
The term 'Jalopy' was a derogatory one commonly used to (in fun) insult someones pride-n-joy, at least that's how it was used when I was a kid..

Definitions of jalopy on the Web:
bus: a car that is old and unreliable; "the fenders had fallen off that old bus"
http://wordnet.princeton.edu/perl/webwn

http://www.fact-archive.com/dictionary/Jalopy
jalopy
An old, dilapidated automobile.

A few minutes searching the web concludes that the actual origins of the word are generally unknown.

"Yiddish is a candidate with shlappe, a term for an old horse that actually derives from Polish. A French origin has also been asserted, from chaloupe, a kind of skiff, though why the name should have come ashore in the process of changing languages is not explained. A lovely theory has it that the word comes from an Italian-American pronunciation of jelly apple. The story goes that a jell ’oppy was one of the decrepit old carts from which Italian immigrants sold this delicacy during the early part of the twentieth century. Others argue that it has a link with the Mexican town of Jalapa, where old vehicles were sent to rest and recuperate. Actually, a Spanish origin seems likely, but galapago, a tortoise, may be a more plausible suggestion, as a description of the slowness of beat-up old bangers."

Source http://www.worldwidewords.org/qa/qa-jal1.htm