shot in the dark; aimless attempt; crapshoot
Idiomatic expression for a blind, unguided effort with little chance of success. Often used when someone tries something without any clear basis or direction.
See also: 闇夜の鉄砲
Answering with a wild guess is just a shot in the dark.
Searching for the culprit without any information is a crapshoot.
Literally 'a pebble in the dark night'. The image is of throwing a stone in total darkness, where you cannot see the target and have no way to aim. The exact origin is uncertain, but it is a traditional Japanese idiom.