Have you ever been in a discussion with someone who constantly frustrates the point you are trying to make by unfairly changing the rules of the argument to fit their side of the discussion? That action is called moving the goalposts, and it is this week’s idiom. If you think about it, the unfairness of this tactic becomes clear when you picture someone moving the goalposts during a game to give their team an advantage; no matter how close you get to scoring a goal, the opposing team will not let you.
This expression originates from British English and according to the OED, one of the first uses of the phrase was in 1924. ‘Moving the goalposts’ is also considered a logical fallacy in which, according to Wikipedia, “evidence presented in response to a specific claim is dismissed and some other (often greater) evidence is demanded. That is, after an attempt has been made to score a goal, the goalposts are moved to exclude the attempt.”
Next time you find yourself in a discussion in which the aim or goal is constantly being shifted so that you can never reach it, the best approach would be to recognize that the other person is not arguing in good faith and you should leave it alone and do something more worthwhile instead.