On the quiet
Definition 1 of 1
Pro Tip 1/3Sensitivity Required
Use when discretion is important; revealing actions or plans could be harmful or spoil a surprise. 
They recruited new team members on the quiet to avoid alerting their competition.
Pro Tip 2/3Not Underground
Does not imply illegal activity, just actions done without public knowledge. 
They donated a lot of money to the school on the quiet.
Pro Tip 3/3Subtle Distinction
More secretive than simply 'quietly'; it suggests almost no one knows, not just being low-key. 
He's been writing a novel on the quiet, almost nobody knows.