Moving the goalposts
Etymology
The phrase borrows from field sports — football and rugby — where moving the goal after a kick makes scoring impossible. It entered general use in the mid-twentieth century as a metaphor for changing agreed criteria mid-contest. The unfairness is in relocating the target, not in the difficulty of the shot.
noun phrase · /MOOV-ing thuh GOHL-posts/
Demanding new, tougher evidence once the original standard has already been met.
What it names
Spotting it stops a debate where no proof is ever enough because the target shifts each time you reach it.
Example
First he wanted one study, then five, then studies from a different decade — he kept moving the goalposts.
Related idea
It resembles the no true Scotsman, which redefines terms rather than evidence to keep a claim safe.