One of the long-running character gags in Eiichiro Oda's One Piece is Sanji's love of women and how far he'll go in order to get any kind of attention from them. This is especially (and hilariously) true of his fellow Straw Hats Nami and Robin. He often refers to them with much gentler tones than the other members of the crew, for obvious reasons, and one fan in Japan even asked Oda about Sanji's exact way of referring to the two.
When asked about the honorifics Sanji attached to Nami and Robin, Oda revealed a surprising fact that Sanji's more gentlemanly side and shows he has a great respect for the two that goes far beyond what fans had thought of before.
In a recently resurfaced questionnaire with Eiichiro Oda for the series, one fan has asked "Ever since I started watching One Piece, I've been wondering why Sanji calls Nami 'Nami-san,' and Robin 'Robin-chan' in Japanese? Robin's the older one, so she should get more respect!! Is there a deep meaning to this?"
Oda's response was just as quirky as you would expect too, "I've never really thought that deeply about it. Recently, one of my staff members asked an interesting question. He said that was women get older, they like to be treated as they're younger, and they like to be treated older when they're young. He asked if Sanji realized that, and was acting accordingly. So I said 'Y-yes, absolutely!'"
So it turns out that Sanji has been far more receptive to Nami and Robin's feelings than he had let on, and adds a bit more playfulness to his usual overbearing romantic attacks on them. But as Oda hilariously admits, this happened to pass completely on accident and thus Oda just rolled with the punches. But that flexibility is why fans love the series so much!
Eiichiro Oda's One Piece first began serialization in Shueisha's Weekly Shonen Jump in 1997. It has since been collected into over 80 volumes, and has been a critical and commercial success worldwide with many of the volumes breaking printing records in Japan. The manga has even set a Guinness World Record for the most copies published for the same comic book by a single author, and is the best-selling manga series worldwide with over 430 million copies sold. The series still ranked number one in manga sales in 2018, which surprised fans of major new entries. As for Avengers: Endgame, you can still find the film dominating theaters around the world as it inches closer to the highest grossing box office performance of all time.