Qitzur Shulchan Arukh – 63:3
Someone who has an insulting nickname, even if he is already used to that nickname and isn’t embarrassed by it, if someone’s intent is to embarrass him, one may not call him by this nickname because it is hurtful speech.
Calling someone a nickname that if taken literally would be insulting, but neither the person it refers to nor the person using it still think about what the term really means is permissible.
When it comes to trying to give offense, as should be obvious, saying “everyone does it” is no excuse. Nor is insulting someone just because you know they’ve learned to ignore this particular insult.