A cacophemism is another word for a dysphemism, the use of a derogatory word or phrase to replace a more neutral original.
A nickname that one does not like being called is known as a "cacophemism." This term refers to a derogatory or unflattering nickname.
I believe that you are quoting the use of something called a "knife cacophemism", the use of a violent metaphor. These knife metaphors have regularly been used in the British media, for example Daily Mail July 2nd 2001 front-page headline "Knives are out for Gordon Brown". The problem with knife cacophemisms is that many youngsters of the age of about 15/16 do not realise that they are metaphors, and may interpret such a phrase as a call to get knives out. This leads to "knife culture", which decent hard-working people are doing everything they can to stop. The Daily Mail newspaper printed "After a week to forget, the knives are out" on May 1st 2009, and the Associated Newspapers Ltd. newspaper group has used knife metaphors on well over 150 occasions during the analysis period of between July 2001 and June 2009. Another example of an alternative knife cacophemism is that used by the Sun newspaper on May 13th 2008. On that day their page 2 headline was "MPs queue up to knife Brown". That afternoon Stephen Bigby was murdered with a knife in Oxford Street, London. Despite - recently reported - a knife crime of some sort every 24 minutes in the UK - much of the media have continued to use knife cacophemisms such as "knives are out.." despite the evidence that they lead to bloodshed (due to literal misinterpretation, and also in the case of the seriously mentally disturbed who can take these kind of headlines as catalytic triggers to commit their crime).