"Gratuitous insult" refers to an unnecessary and unwarranted comment or remark that is made to intentionally offend, belittle, or hurt someone without any valid reason. Full definition
«On a blog that commonly attracts over 500 comments on a thread, cluttering up the thread with gratuitous insults that are not associated with any kind of argument is not what I want this blog to be...» — JC (judithcurry.com)
Gratuitous insults against transexuals justs make you sound stupid. (nystateofpolitics.com)
«To add gratuitous insult, when you go to my page and pull up one of my books, a big pop - up window emerges suggesting I might enjoy another book (not by a Hachette author) at a better price.» (rmfw.org)