A proverb-like expression criticizing after-the-fact opinions — similar to 'hindsight is 20/20' — meaning anyone can claim to have known the right thing after learning the outcome.
試合に負けてから戦術を批判するのは下衆の後知恵だ。
Criticizing the tactics after losing the match is just hindsight from fools.
彼の成功を見てから『やっぱりそうするべきだった』と言うのは下衆の後知恵だよ。
Saying 'we should have done that' after seeing his success is just hindsight criticism.