To betray a confidence by disclosing private information(verb)
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Use "kiss and tell" in a sentence
"No one dreamt of going into hysterics over the veining of a leaf, or penning a rhapsody on the outline of a rain-cloud; nor could it yet be said that, 'if everybody must needs blab of the favours that have been done him by roadside, and river-brink, and woodland walk, as if to kiss and tell were no longer treachery, it will soon be a positive refreshment to meet a man who is as superbly indifferent to Nature as she is to him.'"