SAY NOTHING, keep quiet, don't breathe a word, don't tell a soul, keep it secret, keep it to yourself, keep it under your hat; informal don't let on, keep shtum, don't let the cat out of the bag.
2192; headword
MUM'S THE WORD
Meaning of MUM'S THE WORD in English
Concise Oxford thesaurus English vocabulary. Краткий оксфордский словарь английского языка тезаурус. 2012