secretly *
in secret
on the Q.T.
on the QT

You think I'm the first girl ever to meet a man in secret?

In secret they had then spent a number of hours learning to use them.

He actually began learning at his father's school, in secret.

The two are said to even have married in secret.

And they were allowed to do most of their work in secret.

A project this big could not be done in secret.

Why else would I run away from the house in secret, if not to meet one of my own kind and pass on information?

If they could do that in secret, then almost anything was possible.

The young man had been born to her in secret.

They must for now sit at our Table in secret.