( saying ) used for saying that a situation may still change, for example that a contest, election, etc. is not finished yet, and sb still has a chance to win it
IT'S NOT OVER UNTIL THE FAT LADY SINGS
Meaning of IT'S NOT OVER UNTIL THE FAT LADY SINGS in English
Oxford Advanced Learner's English Dictionary. Оксфордский английский словарь для изучающик язык на продвинутом уровне. 2005