Verb: an act of placing yourself into a conversation/situation that you weren't invited to by standing very closely to people, or the group of people.
Noun: He is standing there clownshoeing that guy.
You are talking to a guy or girl and the clownshoer comes over and stands between you, as if they are wearing clownshoes.
Tony: Dude, I was standing next to the hot chick at the bar, chatting it up, when Mike started clownshoeing me and started talking to me.
Scott: Dude, what a clownshoer.