1
general::
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]
American Heritage Idioms
2
general::
Phrase(s): a shot in the dark
Fig. a very general attempt; a wild guess. • It was just a shot in the dark. I had no idea I was exactly correct. • Come on, try it. Even a shot in the dark may win.
McGrawhill's American Idioms And Phrasal Verbs