Also refers to a car customized with things like
subwoofers, chrome wheels, and racing logos. Typical use of slang is a wannabe-ghetto kid going "Yo, we should jack that shit up" upon seeing a car they would enjoy customizing.
Can also refer to anything which is tweaked out like a "jacked up Athlon box with Geforce2" which would be overclocked like nobody's business, loaded with fans and prolly in a nice custom case to boot.