Keep shtum

I was just listening to the latest episode of 'The Archers' – a radio soap opera from England – and it finished with a character saying "So let's keep shtum about it."

http://www.bbc.co.uk/radio4/archers/

What did he mean? Well, this is a German word being spoken in English – 'stumm' means silent or mute.

And the phrase therefore means: "Let's keep quiet about it." or "Don't tell anyone about it."

Here's the dict.cc entry:
http://www.dict.cc/englisch-deutsch/to+keep+schtum++shtoom++schtoom+about+sth.html
and an interesting internet debate about the phrase
http://forum.wordreference.com/showthread.php?t=1019746

The phrase probably didn't come into the English language directly from German. Instead, it is an example of 'Yiddish' – a dialect spoken by Jews, which is still used by Jewish people in many countries:
http://simple.wikipedia.org/wiki/Yiddish

As well as 'kindergarten' and 'abseil' there are lots of German words that we use in English:
http://en.wikipedia.org/wiki/List_of_German_expressions_in_English