From The Collaborative International Dictionary of English v.0.48 :
Unturned \Un*turned"\, a. [Pref. un- + turned.]
Not turned; not revolved or reversed.
[1913 Webster]
To leave no stone unturned, to leave nothing untried for
accomplishing one's purpose.
[1913 Webster]
[He] left unturned no stone
To make my guilt appear, and hide his own. --Dryden.
[1913 Webster]
From WordNet (r) 3.0 (2006) :
unturned
adj 1: not turned; "left no stone unturned" [ant: turned]