lit. flip over a pig's stomach, meaning one looks fine on the outside, but can turn ugly or show the real evil side in an instant
Definitions (粵典–words.hk)
語句
歇後語:就係屎。比喻兩個原本親密嘅人或團體,一反面就顯出醜陋嘅個性。豬肚指豬胃,外表洗得再乾淨,但一反面時就會見到裏頭好污糟。 to find out a friend's true evil, disloyal nature, to experience how friends turn into enemies after disputes or conflicts of interest.