nim is a Korean honorific suffix used to address someone of a higher status who displays a significant amount of skills and intellect. This honorific is often used by trainees in South Korean music companies to address their CEO and producers, so fans had taken this and applied it to fans who have expertise that not all fans have (knowing Korean, taking high quality photos of idols all the time, etc.), calling them “masternims”.
by Seokjin’s Butterfly October 10, 2018