a flasher, a man who exposes his genitals in public [colloquial]
Definitions (粵典–words.hk)
名詞
露體狂，喺公眾地方向其他人展示下體以獲得快感嘅變態男人，因為通常會淨係著件乾濕褸，因而得名。 a sexually perverted man, especially a man who exposes his genitalia to women in public places, a "flasher"; literally: raincoat man.