Slam dunk
Etymology
In basketball, to "dunk" is to push the ball down through the hoop from above, "dunk" coming from Pennsylvania Dutch for "to dip." Broadcaster Chick Hearn is credited with coining "slam dunk" for the emphatic version. Because the shot almost never misses, the phrase came to mean any near-guaranteed success.
noun · /SLAM-dunk/
An outcome so certain and easy that success is effectively guaranteed.
What it names
The rare case of a sure thing — an action that cannot plausibly fail.
Example
The lawyers called it a slam-dunk case, then lost on a technicality.
Related idea
A slam dunk is treated as guaranteed, while a long shot is the opposite — an attempt unlikely to come off.