Today it can be any car or vehicle used as personal transportation. But it was originally used to refer to classic 70's or 80's era large American cars (impalas, new yorkers, towne cars and so on...), usually found in the ghetto and in poor condition, that has been "pimped out" with garish paint, animal print interiors, wheels too large or too small and so on. They are often roomy, comfortable worn-out gas-guzzlers.
a pice of crap, usually a "car" that has two or more of these qualitys
(1) been crashed and never been to a body shop,
(2)rusted everywhere,
(3) painted with a paintbrush,
(4) home-drawn flames on the side,
(5) has faggidy rims,
(6) a wooden spoiler made in the back yard and nailed to the trunk,
(7) has racing numbers even though it couldnt be possible to race,
(8) a fake engine blower nailed to the hood
my dad's neihbor is a fool and has a MAJOR hoopdie my dad calls it the hoopdy-mobile