to sieze / pounce on someone's flaws and turning that against them; to exploit someone's shortcomings; to use one's own words to against them (usually by taking it out of context)
Definitions (粵典–words.hk)
動詞
比喻攞人哋講過嘅嘢嚟攻擊返佢 to use someone's words in support of an attack against them; to pick up on what someone has said before and use it to attack them in a new set of circumstances; literally: to pull the back leg