不明所以 [－－－－]

Jyutping bat1 ming4 so2 ji5
Pinyin bù míng suǒ yǐ

Definitions (粵典–words.hk)
  1. 語句
    無辦法​理解​、​完全​唔​知道​點解​要​咁​做
    nonsensical
Definitions (Wiktionary)
  1. phrase, idiomatic
    to not understand the reason why