as close (/dumb)as wax (/an oyster); as mum as a mouse; breathe not a single word; button up one’s mouth; hold one’s tongue; keep a silent tongue in one’s head;keep one’s mouth tightly shut; keep one’s tongue between one’s teeth; keep one’s mouth as tight as a jar;be prudent and reticent about; seal one’s lips; as secret as the grave;tight-lipped; tight-mouthed ❍ 富郑公有“～，防意如城”之语。(周密《癸辛杂识别集下·守口如瓶》) Fu Zhen-gong once said，“Keep your mouth as tight as a jar and guard against selfish desires as if guarding a city. ”
守口如瓶
keep one’s mouth shut; have one’s lips sealed;be tight-mouthed
守口如瓶shǒu kǒu rú pínɡ
守口：紧闭住嘴不说。形容说话非常谨慎，也形容严守秘密。keep one’s mouth shut, hold one’s tongue, seal one’s lips, as close as an oyster, secret as the grave, keep a calm sough