put/stick the knife in

put the knife in

To 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 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.
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

put/stick the ˈknife in

,

put/stick the ˈknife into somebody

(informal) criticize somebody or deliberately try to harm them: The senator has quite a few enemies that would love to stick the knife in if they got the chance.
See also: knife, put, stick