it is used to describe someone who is opinionated and self-righteous
Definitions (粵典–words.hk)
名詞
不自量力、自視過高嘅人;亦用嚟批評人要反省自己嘅水平 used to put someone down for thinking too highly of themselves or do not know their place, i.e. "who does he think he is?".
你當你乜嘢新絲蘿蔔皮?
你当你乜嘢新丝萝卜皮?
nei5 dong3 nei5 mat1 je5 san1 si1 lo4 baak6 pei4 Who do you think you are?
參看
新鮮蘿蔔皮、身屍蘿蔔皮
Definitions (Wiktionary)
phrase, Cantonese, idiomatic
someone who overestimates one's capabilities
你乜嘢新丝萝卜皮呀?
你乜嘢新丝萝卜皮呀?
nei5 mat1 je5 san1 si1 lo4 baak6 pei4 aa3? Who do you think you are? (used to put down someone who is too arrogant)