If you translate the word stupid into Korean, then it is 바보, 멍청이. To someone who acts ridiculous, you can say "너 바보같아" To something ridiculous, "이거 이상해" In Korean culture, it can be a little bit offensive if he/she is not your close friend, so be careful when you use it.
@armybabie Ah-ha! That's okay. If you say it with smile and soft voice then they will notice that it is a joke. When you say it online, some wavy marks(~) and ㅋㅋ(Lol) will make the mood softer. "바보~~ㅋㅋㅋ" like this.
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