Undisclosed means not made known publicly. An undisclosed sum, undisclosed reasons, an undisclosed location.
For an undisclosed sum is the standard formula in reporting a deal nobody will confirm the price of. Failing to disclose something you were obliged to is a separate and more serious matter.
1adjectiveundisclosedNot made known publicly.
“The singer is recovering in hospital from an undisclosed illness.”
undisclosed is a quality.
From un- + disclosed.
Etymology from Wiktionary, CC BY-SA