까먹다 (kkameokda), forgetting completely.
The word is slangy and informal, mostly used in spoken language.
Examples
어제 들은 이야기 다 까먹었어. → “I forgot everything I heard yesterday.”
비밀번호를 또 까먹었네. → “I forgot my password again.”
중요한 발표 날짜를 까먹어서 큰일 날 뻔했어! → “I forgot the important presentation date, so I almost got in trouble!”
See also 깜빡하다