The story
A metaphor of British origin derived from association football (soccer), from the idea that it is difficult for a player to kick the ball into the goal if it is moved.
Etymology from Wiktionary, CC BY-SA 4.0.
move the goalposts
move the goal·postsToday it means: To alter the terms of an agreement or an agreed target, or the rules of a negotiation while it is ongoing, especially in an unfair way. Full definition →