Zoro has this disgusting addiction to the number three: three swords, three earrings, Sanji...
I'd like to believe it drives him crazy how much Sanji's name fits in his 3s obsession, he has given it much thought because from the first moment they met he thought its such a cool name! but yet again he would never hear the end of it from the cook if he knew, so his tsundere ass refuses to ever call him by name.
Which actually works because I think deep down, Sanji resents his own name. He was nothing to his father more than "son number 3" and instead, Sanji uses nicknames with the people he cares about (He never calls Zeff by his name, it's always "shitty geezer") so really, when he calls Zoro "marimo", it's not really an insult but a term of endearment. And he has grown to accept "shitty cook" as the same kind of treatment. Its all unspoken and problematic but that's them.
Anyways im so soft about them being literally written for each other...
