Schtum means quiet — not making a sound, or not telling what you know. You almost always hear it in keep schtum (also spelled keep shtum): stay silent; do not talk about this. It is British informal talk, a bit cheeky, the kind of thing you say when a secret should stay a secret. You would not put it in a school essay. People also say go schtum: suddenly stop talking. Spellings wander — schtum, shtum, shtumm — because English borrowed the word by ear. It comes from Yiddish, where a close word means mute or silent, sitting next to German stumm, “mute.” The feel in English is “don’t you dare say a word.”

（英式口語，無固定譯名）閉嘴不說、保持沉默。幾乎都出現在 keep schtum。