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. . Others argue that it has a link with the Mexican town of Jalapa, where old vehicles were sent to rest and recuperate.