slang for backlight; to do something with one's back to the light; the light shining in the wrong direction, covered by the shadow of one's hands when writing
Definitions (粵典–words.hk)
形容詞
背光、寫字嘅時候手影落喺紙上要寫字嘅地方 to have light from behind, causing one's hand to cast a shadow onto the paper where one is writing