"Clown car" (or "clowncar") is the nickname for the Soviet BA-64 vehicle in the realistic war game Red Orchestra: Ostfront 41-45. Coined by an unknown community member, the term spread like wildfire. The reason why many people call it the clown car is because the vehicle is often used in favour of unrealistic and "gamey" tactics. Such tactics include the popular drive-by-satchelling (wich envolves racing towards a tank, instantly jumping out at the very last second, dropping an explosive satchel charge on the back of the enemy tank, then quickly driving off before blowing up the tank).
Such "tactics" are frowned upon by many RO players (especially those who enjoy realistic tactics), but it is also considered fun to do every once in a while by quite a lot of people.
"K guys let's try to pile as many soldiers as possible on this clown car and open up a can of pwnage on them lolol!"
"We should name the BA-64 the Clown Car in honor of all the brave clown satchel-throwers serving for their Motherland!" -- Feldwebel Crin
"the Tigerpolizei are rounding up BA64 drivers so look out! :-O" -- middenrat