褲門 [裤门]

Jyutping fu3 mun4
Pinyin kù mén

Definitions (Wiktionary)
  1. noun, slang
    fly (strip of material at the front of a pair of trousers, pants, etc.)
    (syn.) 天安門, 拉链, 褲子的拉鏈, 拉鏈, 天安门, 裤子的拉链