What do you call a desperate attempt unlikely to succeed? For example, when other ideas have failed and you have one final go before giving up
Forlorn hope - a victorian British army expression for a group of men who would storm a breach in a wall. Any one who survived would get promoted or a reduction in punishment. As the name suggests it was very unlikely to succeed.
I think that a good way to say this is "a shot in the dark." It's defined here as "an attempt that has little chance for success."
It's idiomatic. It refers to firing a gun (or shooting an arrow) when you can't see the target. I think it contains the desperation and unlikelihood that you want.
Such things are often called "a last gasp".
Grasping at straws (Cambridge dictionary)
In the game of Chess, when a piece is threatened and cannot be saved, it can often still be used to capture one last opponent piece or pawn of lesser value before it's taken.
Normally, you wouldn't want to exchange one of your pieces against a piece of lesser value; but since the piece cannot be saved anyway, that capture is its final throe.
Such a move is called a desperado.
By analogy, any last attempt before giving up could be called a desperado.
A last (or final) roll of the dice
A million to one chance, or a one in a million chance, while not single words are common phrases used to describe making an attempt against desperate odds. In fiction of course such plans always work. It is such a well known plot device that it was parodied in Terry Pratchett in several books.
Scientists have calculated that the chances of something so patently absurd actually existing are millions to one. But magicians have calculated that million-to-one chances crop up nine times out of ten.
I mean, it's a good job we've got a last desperate million-to-one chance to rely on, or we'd really be in trouble!
One could also call it a quixotic attempt.
- "Quixotic" does mean unlikely to succeed.
- It doesn't imply a last attempt, your first attempt can be quixotic.
- It additionally means idealistic.
You can use it to describe an attempt to do something e.g. "They made a quixotic lunge for the fleeing cat.". Or it can equally describe a person e.g. "The quixotic scholar was hard at work and learning nothing."