It slipped my mind.
→   깜빡했지 뭐야.

[예문]
 ※  slip one’s mind :잊어버리다.깜빡하다.

A :  Did you return the video tape?
     비디오테입돌려줬니?

B : What am I gonna do? It completely slipped my mind.
       어떡하지? 깜빡했어.