出典:Wiktionary
Probably from keep the ring. In days past when spectators would encircle participants in a prizefight or a performance, people would be employed to maintain order among them and keep them from coming too near the participants.[1]
hold the ring (三人称単数 現在形 holds the ring, 現在分詞 holding the ring, 過去形および過去分詞形 held the ring)