A jalopy is an old car that isn’t working very well. You’d never call a new, smooth-running car a jalopy.
There are a lot of insulting words in English, mostly for people. This insult is for a car: a jalopy is a rundown, beat-up, falling apart car that needs to be replaced. You’ll never see a car salesman say “Check out our selection of jalopies!” A jalopy might have been a great car at one point, but it has seen better days.
Thanks to Vocabulary.com