There are many way to say" silly". 1. 너 바보지?! Only with friend 2. 어리석게 굴지마! we dont speak that often. but for writing is okay. 3. 너 왜 그래?! This is close that "are u silly".Meaningly.

How to say Silly meaning in a funny way in Korean? 어리석다 to be goofy.

http://endic.naver.com/enkrEntry.nhn?sLn=en&entryId=1465bf3fecf241f1b9dd2f16d245102e&query=silly Check the link I gave - it has many definitions/uses for the word silly in Korean. You have to be careful in using those words though, because Koreans might find them offensive >_<