arrow shot with the target unaware of/a cunning stance taken to harm an unaware one
Definitions (粵典–words.hk)
名詞
原意指趁敵人唔留意嘅時候向佢發箭攻擊。宜家通常係指喺人哋唔為意嘅時候，或者喺人背後，作出惡意攻擊，令對方受傷害。 Originally, it meant to shoot an arrow towards the opponent when he/she was not paying enough attention. Nowadays, it means taking actions intentionally to cause damage to someone, especially when he/she is unaware of the actions.
大偉個新同事，表面同佢好似好老友，但係不停喺佢背後放冷箭。最慘係佢老細誤信讒言，炒鬼咗佢。唉！
大伟个新同事，表面同佢好似好老友，但系不停喺佢背后放冷箭。最惨系佢老细误信谗言，炒鬼咗佢。唉！
David's new colleague appeared to be very friendly with him. But he kept doing all sorts of things to undermine his position in the company. The worst is that David's boss actually believed the false accusations and sacked David. Sigh!
David's new colleague appeared to be very friendly with him. But he kept doing all sorts of things to undermine his position in the company. The worst is that David's boss actually believed the false accusations and sacked David. Sigh!
Cantonese
先排有人向佢放冷箭，爆晒佢贪污啲秘闻出嚟。
先排有人向佢放冷箭，爆晒佢貪污啲秘聞出嚟。
Someone has just shot a sniper arrow at him, all the bit and pieces about his corruption case is spreading around.