put the knife in

put the knife in

To be, do, or say something especially critical, unpleasant, or mean-spirited, especially to someone who is already vulnerable or weak. Primarily heard in UK. John seems genuinely remorseful for what happened; there doesn't seem to be any reason to put the knife in at this point. And then he really put the knife in by telling me that he'd only pretended to like my writing. You would think the rise of cheap electric cars would put the knockers on Big Oil's profits, but they actually seem to be embracing the change.
See also: knife, put

stick the knife in

To be, do, or say something especially critical, unpleasant, or mean-spirited, especially to someone who is already vulnerable or weak. John seems genuinely remorseful for what happened; there doesn't seem to be any reason to stick the knife in at this point. And then he really stuck the knife in by telling me that he'd only pretended to like my writing.
See also: knife, stick

put the knife in

or

stick the knife in

mainly BRITISH
If someone puts the knife in or sticks the knife in, they deliberately do or say things which will upset another person or cause problems for them. Every time he applied for a job, someone put the knife in and made sure Jack's background became known. BBC colleagues — often eager to stick the knife in — defended her yesterday.
See also: knife, put
References in classic literature ?
Let me introduce the topic, Handel, by mentioning that in London it is not the custom to put the knife in the mouth - for fear of accidents - and that while the fork is reserved for that use, it is not put further in than necessary.
Gary McAllister put the knife in on the team he helped take to the League title yesterday.