跑馬射蚊鬚 [－马－－须]

Jyutping paau2 maa5 se6 man1 sou1
Pinyin pǎo mǎ shè wén xū

Definitions (粵典–words.hk)
  1. 語句
    要​喺​跑​緊​嘅​馬​上面​射飛​緊​嘅​蚊​鬚​，​形容​非常​之​困難​，​冇​把握​，​機會​十分​渺茫
    so difficult that one has no confidence; with very low chance; unlikely