The truth is, dictionary makers have not the slightest idea where jalopy comes from. It was spelled all sorts of ways when it first appeared, a sure sign that oral transmission came first. So, in lieu of facts, here are some of the stories that others have recounted in worthy attempts to make some sense of the matter. 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. I give no guarantees about any of this. We have to leave it as one of life’s mysteries. (this was found on http://www.worldwidewords.org/qa/qa-jal1.htm)

The noun "jalopy" has 1 senses. 1. bus, jalopy, heap -- a car that is old and unreliable; "the fenders had fallen off that old bus"

If I remember correctly, it was Desperate Dan who first said it, so maybe he made it up.