A slur towards the Japanese group of people, Nakomonkeys. People can only use Sinbmen if they are apart of the group. The Nakomonkeys are a small Men hating & alternatively dressed group, who was very much hated by young local school children thus them calling them a Sinbmen, (Sin Man) or 今月の少女, which is now a slur.
Pajama casual is a term used when working from home and preparing for a video conference. All business above the waist and your pajama pants down below.