A 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 Bros. 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.
Clown cars can also be used to describe vehicles in computer games that house an unrealistically large number of occupants. They have also been used to describe political contests with a large number of candidates.
References
Clown car Wikipedia(Text) CC BY-SA