a long shot
A long shot can mean many things. In any game of sport that requires hitting a target or scoring a goal (such as golf, basketball, darts etc), a long shot means trying to hit the target from a long distance. A ‘hole in one’ is a very long shot. So in everyday life the expression long shot means ‘something with only a very small chance of succeeding’. For example, I would like to win the lottery. I know that winning the lottery is a long shot (a *very* long shot!), but I still try. Sometimes long shots work!
A: I know you really like Tarou. Why don’t you ask him on a date?
B: Oh, he’s so good-looking... I can’t believe he’d be interested in me.
A: You won’t know until you try!
B: You’re right. I know it’s a long shot, but I should try!
In horse-racing, a horse at 100:1 is a long shot. Have you ever gambled on a long shot?