Meaning of LET SLIP STH in English

LET SLIP STH

to give sb information that is supposed to be secret :

I happened to let it slip that he had given me £1 000 for the car.

She tried not to let slip what she knew.

Oxford Advanced Learner's English Dictionary.      Оксфордский английский словарь для изучающик язык на продвинутом уровне.