Biography link is on the QT

Did you read the featured life of the day from the Oxford Dictionary of National Biography? Meet Charlotte Louisa Collins:

"A smart and stylish girl you see
The belle of good society
Fond of fun as fond could be
When it's on the strict QT."

But what does QT mean? Or, what did it mean before it meant "cutie" on T-shirts today? Read word origins for a surprise.