It was originally used by goldwater or goldstein(Gold something, I can't remember his name) who was an opponent of the theory proposed by Lemaitre. It was said on a radio show as an oversimplification in a way sort of like, "That can't be possible, some sort of ridiculous big bang!" but the name stuck. The theory itself was proposed by Lemaitre, a priest. It was termed the big bang because Lemaitre described it as a large explosion from which all matter expanded from, hence a big bang.