The clown car is a prop in a circus clown routine. A common example of such a routine involves an implausibly large number of clowns emerging from a very small car, to humorous effect. The first performance of this routine was in the Cole Brothers Circus during the 1950s.
The clown car is frequently used as a comedic gag in film. One is featured prominently in Mel Brooks' remake of To Be or Not To Be.