to tell a secret carelessly or by mistake :
I wanted it to be a surprise, but my sister let the cat out of the bag.
to tell a secret carelessly or by mistake :
I wanted it to be a surprise, but my sister let the cat out of the bag.
Oxford Advanced Learner's English Dictionary. Оксфордский английский словарь для изучающик язык на продвинутом уровне. 2005