shǒu​kǒu​rú​píng

tight-lipped

守口如瓶 shǒu​kǒu​rú​píng
tight-lipped idiom
to keep one's mouth shut and maintain strict secrecy; not breathing a word about something

He is tight-lipped about his past and never mentions it to outsiders.

Frequency

Movie subtitles
2.83 per million appears in 1% of films
Written text
0.547 per million

Sources

Definitions
CC-CEDICT