What does “bêtise” mean in French?

Une bêtise is built on bête (“stupid”, also “animal”) but it lands much more gently than that. Applied to a child it means mischief: something silly rather than wicked. Faire une bêtise is knocking the paint over; ne fais pas de bêtises is the line called after a kid leaving the room. It covers roughly the ground English gives to “that was naughty”.

Said of an adult the word hardens — faire une bêtise can mean acting recklessly — and dire des bêtises shifts to talking nonsense. In this clip a parent reaches for it after a toddler throws something at the wall: c’est une bêtise. Note how the reproach is framed impersonally — on ne jette pas..., “one doesn’t throw...” — which is how French adults often state a rule to a child rather than pointing at them with tu.