The Origin of the expression "Mum's the word"
You can also use "telling you this on the QT"
And the origin of that expression is as follows:
According to Robert Hendrickson, in The Facts on File Encyclopedia of Word and Phrase Origins, the first reference is from a British ballad of 1870, which contained the line “Whatever I tell you is on the QT”. It seems to have been just an abbreviated spelling, using the first and last letters of the word quiet, the mild obfuscation also suggesting a meaning for the expression. The Oxford English Dictionary has a first sighting from 1884: “It will be possible to have one spree on the strict q.t.”. Mr Hendrickson points out that it also occurs in a famous London ditty of 1891, sung by Lottie Collins, which also introduced the famous chorus line “Ta-ra-ra-boom-de-ay”:
A sweet Tuxedo girl you see,
Queen of swell society,
Fond of fun as fun can be,
When it's on the strict Q.T.