1
general::
Betray a confidence, as in A real lady doesn't kiss and tell. This idiom originally alluded to betraying an amorous or sexual intimacy. First recorded in 1695, it is still so used, as well as more loosely, as in Don't ask how I voted; I don't kiss and tell.
American Heritage Idioms
2
general::
Phrase(s): kiss and tell
Fig. to participate in something secret and private, and then tell other people about it. (In actual use, it usually refers to a person of the opposite sex even when it does not refer to actual kissing.) • The project was supposed to be a secret between Jane and me, but she spread it all around. I didn’t think she was the type to kiss and tell. • I am willing to discuss it with you, but only if you promise not to kiss and tell.
McGrawhill's American Idioms And Phrasal Verbs