Camie, the mermaid, calls Luffy "chin" as a unique and consistent part of her personal speech pattern. This distinctive suffix, "Chin," is appended to names by Camie in a fashion similar to how Japanese honorifics such as "San" or "Chan" are used to address others.
Camie is known for her peculiar verbal habits. For example, instead of simply saying "human" or "humans," she typically uses the phrases "human person" and "human people," respectively. Her application of "Chin" to names, including Luffy's, is another manifestation of this personalized speaking style, making it a hallmark of her character rather than a specific attribute or nickname for the individual she is addressing. It’s simply her characteristic way of interacting with and referring to others.
