A wild, unsubstantiated guess; also, an attempt that has little chance for success. For example, It was a shot in the dark, but the engineers had a hunch that replacing the valve would make the system work, or You can try looking for your key on the beach, but I think it's a shot in the dark. [Colloquial; late 1800s]
The American Heritage® Idioms Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company.