Old, weird term used to describe a professional wrestling ring. If professional wrestling indeed developed from amateur wrestling, it may make sense, as amateur wrestling is performed on a mat with a circle on it. Step over, and you're out of bounds. Perhaps in the transition to the more available boxing ring-style venue developed, this hazy memory of the actual sporting basis underlying professional wrestling manifested in early professional wrestling announcers to come up with the term.
There is no circle involved in professional wrestling currently. Not many people call the ring a squared circle anymore, either.
other sports that take place in circles:
other sports that take place in squares: