어불성설이 · 어불성설을
nonsense; an argument that doesn't hold up
Example
아무 노력 없이 성공하겠다는 것은 어불성설이에요.
Expecting to succeed without any effort is nonsense.
Word parts
어 words+불 not+성 become+설 argument
Literally 'the words don't make an argument.'
Good to know
Formal and argumentative; in casual talk people just say 말도 안 돼요.