a shot in the dark(WILD) GUESS, surmise, supposition, conjecture, speculation
[TahlilGaran] Dictionary of Contemporary English ▲
shot in the dark n. phr. An attempt without much hope or chance of succeeding; a wild guess.
A was just a shot in the dark, but I got the right answer to the teacher's question. [TahlilGaran] English Idioms Dictionary ▲