originally a reference to a children's toy. the clown (or whatever character-- i had heman) was a weighted bottom blow-up punching bag. it was designed to pop back up to a standing position when knocked over. essentially, punching the clown is a useless act which accomplished nothing. often used in reference to menial tasks and bureaucratic corporate jobs.
i do about fifteen minutes of real work everyday, but mostly i just sit around and punch the clown.