Prior to 2016, a common epithet online used against left-leaning people (or those perceived to be left-leaning) was "libtard". The insult is still used, despite losing ground to other "debate" tactics, but I want to discuss why it's a completely silly term.
First off, the "-tard" part is clearly taken from the word "retarded", which is itself a slur. The implication is that people with left-leaning perspectives are mentally challenged. I don't think I need to explain why it's offensive.
Where the insult is silly lies in the phonetics and spelling. The attack also comes from the word "liberal". There's just one problem: There is no "T" in "liberal" - "libtard" makes zero sense as a result. If anything, the way it's spelled it's more of an attack against libertarians, as there is a T in that word. If anyone wants to confuse a person who cries "libtard", ask what libertarians have to do with the discussion.
In the past year, however, "libtard" has lost out to alt-right (white supremacist) phrases such as "Are you triggered?", "Do you need a safe space?" and "Stop being so sensitive", which are awful (and downright annoying) for other reasons. It's still being used, of course, and anyone who blurts it is often laughed at for being a fool.