똑똑 + 두드리다 / 떨어지다
knock knock (on a door); drip drip
Example
누가 문을 똑똑 두드려요.
Someone is knocking on the door.
Word parts
의성어 (sound word) for light tapping or small drops falling. For drops, the dark-vowel partner 뚝뚝 means bigger, heavier drops.
Good to know
Don't confuse it with 똑똑하다 'to be smart', which is an unrelated word.
Related words