Make every possible effort, use every possible source or resource.
For example, To raise ten thousand dollars to keep the shelter open, we must leave no stone unturned.
This expression alludes to an ancient Greek legend about a general who buried a large treasure in his tent when he was defeated in battle.
Those seeking the treasure consulted the Oracle of Delphi, who advised them to move every stone. The present form dates from the mid-1500s.
